A little background
I use a nice, sturdy tripod for shooting still life photos, and sometimes portraits. More often than not for still life shooting, I use live view either because the camera is at an odd height or angle, or because I'm shooting in very low light that makes it difficult or impossible to compose and focus through the viewfinder.
There are previous questions that ask about whether the mirror flips back down and then up again during live view shooting. In my camera the answer is
yes no, but there's more to be aware of (detail further down).
The Blanston Hypothesis
It seems that if the shutter speed is fast enough, then any vibration of the camera would be insignificant because the image is captured too fast for the camera to move too much during the exposure. And it seems that if the shutter speed is slow enough, the short amount of time that the camera vibrates wouldn't matter because it would be buried below the noise of the capture (assuming very low light, no flash, etc.). So I figure there must be a range of shutter speeds where mirror lock-up makes a difference in image quality. It wouldn't surprise me if it's related to focal length, sort of like the 1/(focal length x crop factor) guideline for non-IS handheld shooting.
So, as the title states, at what shutter speeds is mirror lock-up worthwhile? Is my reasoning correct (or at least sane)?
The detail I promised you earlier
This answer indicates that live view does accomplish mirror lock-up using a Canon 70D, and it appears the same is true for my 80D, but there is an additional menu setting I needed to be aware of: Silent LV mode.
When I use mirror lock-up in normal (non-live view) mode, I can clearly hear that the first curtain noise at the beginning of the exposure is a very minor "tick" sound, which makes sense. If I do this with a suitably long shutter speed (say 1 second or more), I can clearly separate the sounds at the beginning of the exposure from the sounds at the end, when the mirror flips back down.
However, when I use live view, I can very clearly hear noises that I first thought was the mirror moving at the beginning of the exposure. It turns out the mirror isn't moving, but something about the way the shutter activates in this mode created enough noise and vibration to trick me into thinking that's what was happening. When I found the Silent LV option in the menu, it was set to "Disable." Once I changed that to either Mode 1 or Mode 2, the noise and vibration more closely matched what I'd observed when using lock-up and viewfinder shooting.
This question was initially posted because I thought that I couldn't get mirror lock-up in live view mode. It turns out I can, but I'm still curious about the answer.