This may entirely depend on what camera you use. Canon cameras are rather notorious these days for not having competitive dynamic range. They have decent dynamic range, but it is no longer competitive, and there are alternative options that allow you to preserve considerably more shadow detail without the need to either worry about using ETTR at all, or you simply don't have to be as aggressive with ETTR.
For a full explanation on dynamic range, which is a bit of a prerequisite for understanding why ETTR is needed, I've written a comparison article on my site: Dynamic Range
For a quick example, here is an A7r (left) and 5D III (right), both exposed for the same scene such that the brightest highlights were just barely clipped, thus making complete use of the entire dynamic range of both cameras:
(See link above for higher resolution)
The 5D III is already showing more noise with a mere 3-stop push. The A7r holds up well until a full five stop push. I even went "extreme" and pushed the images well beyond five stops (about seven stops for the shadows, with the additional adjustments):
Again, the A7r holds up quite well, the 5D III just doesn't cut it. (Newer Canon cameras have less banding, however fundamentally their dynamic range has not improved much...maybe a third of a stop. You won't see much better performance with a 7D II, a 6D, nor even a 5Ds.)
For reference, here is what the original images looked like strait out of camera (as rendered by Lightroom):
Lot of deep shadows there, some that appear dead black. LOT of information that can be recovered.
A lot of people complain that having more dynamic range in the camera just means you have lower contrast in your images after you lift the shadows. That is a naive reaction, usually one given due to either distinct and strongly held brand loyalties, or simply out of naivete or a lax understanding of post processing. A shadow push is simply the beginning step of a processing workflow. Contrast can be restored to the image post-push:
While you can see some noise in the 7-stop lifted shadows of the A7r here (top), and that 7-stop lift does wash the image out...you should never just stop there. As you can see, the same image with some additional adjustments to restore contrast (bottom), and a little bit of NR, result in very clean shadows, despite the fact that they were buried seven stops deep in the blackness of the original image.
The 5D III, however...despite similar edits, and much heavier NR, it simply doesn't hold up. With the 5D III, despite pushing ETTR to the limits, I was only able to preserve so much shadow detail. Now, without ETTR, I would have lost EVEN MORE shadow detail with the 5D III...and I would have lost nearly as much with a 6D, a 7D II, or a 5Ds (despite all of them having lower dark current and less banding, their read noise is just about as high.) ETTR becomes essential with a Canon camera. If the shadows are very important, clipping highlights even more may be acceptable. With a still scene, HDR would probably be preferable.
Before I can really answer your question about ETTR, my first question is:
What camera do you have?
If you have a Canon camera, then you do need to pay much closer attention to what range of tones you are preserving in your photos. Technically speaking, "correct" ETTR means you NEVER throw away any highlights. There are various schools of thought on that, and in some cases, throwing away some highlights may be acceptable, and if there is shadow detail that is more important than those highlights, the simple logical conclusion is to throw away the highlights. For purists, that would be going beyond ETTR, but in general, I think you do what you have to in order to produce the photo you want...and if that means throwing away highlights, and you are consciously making that decision, then its an entirely acceptable option.
Is there such a thing as ETTL? Personally, I don't think ETTL is a viable concept. We are talking about signals here...to be more specific, we are talking about digital signals and SNR. In a digital signal that is produced by an image sensor, you have a pretty hard cutoff at the point where highlights clip. This is due to the nature of how the signal in each pixel is read out and converted to a digital number. The analog response of the sensor itself may not be linear throughout the entire range of it's literal sensitivity, however most camera manufacturers limit the "usable" range of charge to within the largely linear response, and any signal beyond the range of linear response at the high end is simply clipped to white.
When it comes to shadows...fundamentally, shadows are not clipped, not by any natural consequence of sensor design. Some manufacturers in the past (Nikon and Sony) have chosen to clip a little bit of the blacks, however both manufacturers these days use a bias offset of around 600 now. The key problem with shadows is they get lost in noise.
So when you create a photograph with a digital camera...you have either clipped the highlights, or not. When it comes to preserving highlights, so long as you understand your camera and have a good feel for where it clips, you preserve highlights simply by not clipping them. That does not require ETTL per se...it simply requires paying careful attention to how far the highlights extend in your histogram. (For Canon cameras, adding MagicLantern, and using it's RAW histogram, can GREATLY help in this area.) So long as you don't push your highlights beyond the point where they clip, there is nothing else you need to do to preserve them. They are all preserved. That's it.
The entire reason ETTR exists is because in the highlights, and in the midtones, the signal is strong enough that the SNR is acceptably high. You have low noise because noise is the square root of the signal. The square root of something grows more slowly than that something itself. If your signal is 10,000e-, your noise is 100e-, which also so happens to be a ratio (or SNR) of 100:1. If your signal is 100,000e-, your noise is 316e-, an SNR of 316:1. It's in the shadows where noise can become a problem. A signal of 1000e- has noise of 31e-, which is an SNR of a "mere" 31:1. A signal of 1000e- is actually fairly strong...that is actually probably more like low midtones or brighter shadows in most cases. True shadows can be as little as 50e-, and deep shadows can be as little as 10-15e-! There you have very low SNR.
The problem with shadows is compounded by electronic noise, however. There is noise inherent in every signal, but with a digital camera, you also have added electronic noise. Read noise, as it is usually termed, is compounded with (added to in quadrature) the noise in the signal. If your camera has 30e- RMS read noise, and your signal is 50e-, then your SNR is not 50/SQRT(50), it is 50/SQRT(50 + 30^2)!! That is a minut SNR of 1.62:1! A Canon 5D III has MORE than 30e- read noise at ISO 100. (Which should explain why it's detail in the shadows breaks down and becomes unusable with a mere 3-4 stop push above.)
If on the other hand, you have a mere 3e- RMS read noise, and your signal is 50e-, then your SNR is 50/SQRT(50+3^2), which gives you an SNR of 6.51:1. An SNR of 6.5:1 is actually fairly decent for shadows. (As an astrophotographer, I often work with SNRs smaller than that.) The Sony A7r, and for that matter, the A7 and A7 II, the A6000, the Nikon D800, 810, 600, 610, 750, the 7000 line, the 5000 line since the 5100, the 3000 line since the 3100, etc. all have read noise in the range of about 3e- or less (regardless of ISO). The Samsung NX1 has read noise around 5e-. (There are new sensors being tested that have constant read noise as little as 1.5e- and less at base gain!) If you have any one of these cameras, then careful and meticulous ETTR becomes less of a necessity. ETTR is still recommended, it's just good practice to make the best use of your sensor's dynamic range as you can, but you don't have to be as concerned about pushing it to the utter limits, carefully preserving highlights, while gaining as much signal as possible in the shadows. The shadows will, as you can see from the above, handle SIGNIFICANT pushing in post, and with some effort to restore some contrast to the scene after a heavy shadow push and even a little bit of NR, can largely negate the need to really worry about ETTR beyond generally shifting your exposures to the right...without clipping highlights.
