Regarding the statement: 

> Is lower ISO always better?

There seem to be a variety of opinions on this topic, and while they may seem mutually exclusive, I am not certain that is the case. There is no cut and dry "Yes, X ISO setting is always better." I think which is better is very dependent on context...on what it is you are trying to shoot, and what kind of light is available.

The two points of view expressed so fare include:

 - Use the highest ISO possible to maximize saturation (and therefor minimize noise) without clipping highlights.
 - Use the lowest ISO possible to minimize noise while using the correct shutter and aperture to achieve a proper exposure.

The general consensus is that opting for the lowest ISO possible to achieve a proper exposure is the best approach. That statement is heavily laden with hidden meaning, however, as the lowest ISO ***possible*** may not necessarily be ISO 100. You may be forced to use a higher shutter speed or a narrower aperture to achieve the necessary *creative* effects you desire, forcing you to use a higher ISO to maintain proper exposure. You may also face issues with available light and reach the limits of your lens (i.e. maximum aperture) and be forced to use a higher ISO to achieve a correct exposure. I think that is the point Matt Grum has been trying to make. 

Barring any specific creative needs (i.e. action-freezing shutter speed or large DOF via small aperture), the lowest ISO setting with the necessary shutter and aperture to produce a "correct" exposure (i.e. an exposure that is neither over or under exposed, or if you follow ETTR, one that has not blown out any highlights) is, still, the best practice. This can be easily demonstrated, as the sample shots below show. This set of examples is shot with several things in mind:

 1. There is no requirement for a short shutter speed, so it could be as long as necessary to get a correct exposure.
 2. There is no requirement for depth of field, so the widest aperture will do.
 3. The available light is fixed, and can not be changed.
 4. A tripod and cable release will be used to eliminate any camera shake.

![alt text][1]

The clearest shot is, as you can see, the ISO 100 shot. At maximum aperture, ISO 100 required a 0.6 second exposure time. This is pretty long, but since there was no motion, a lengthy exposure is not an issue. The ISO 1600 shot is still properly exposed, and used a shutter speed exactly four stops faster than the ISO 100 shot. Despite being properly exposed, there is obviously additional noise. The final shot was another ISO 100 shot at the same settings as the ISO 1600 shot, with the exposure value adjusted by +4EV in Lightroom. The noise level in this shot is obviously FAR higher than the other two since it has been digitally enhanced.

Given these examples, we can come to a few logical conclusions:

 - Using the minimum ISO for scenes with no motion when shooting with a tripod will produce the lowest noise possible.
   - Common for landscapes and still life scenes.
   - Uncommon for sports, wildlife, bird, or any other action photography.
   - Depends for things like portrait shots, however since you can usually control the amount of available light for portraits, a low ISO should usually be possible.
 - Using the minimum ISO for a scene that allows you to expose correctly will minimize noise, but may not be the lowest possible noise.
   - If you need to freeze action with a high shutter speed, or increase your DoF with a narrow aperture, you may need to increase ISO to maintain exposure.
 - Shooting with the lowest ISO regardless of the situation is a bad idea.
   - This will often result in under exposure (possibly severe) when available light is not optimal.
   - This will result in considerably more noise when exposure is corrected in post processing.

I think a good way to learn what ISO settings are best for various scenes and lighting is to set your ISO to Auto, use manual mode, take a few shots, and review the results. The camera's automatic exposure metering will always try to create a "proper" exposure, and when you have a scene with a broad range of tones, it should choose the right setting most of the time. You can also try manually setting a higher or lower ISO than the one the camera chooses automatically, and re-take a shot to see the results. Outside of maybe landscape and still life photography, you probably won't find a single "correct" or "best" ISO setting. However, I do believe the general rule of thumb will always be:

> Use the **lowest** ISO you can while
> maintaining a proper exposure for the
> type of shot you are making.

For still life, that will probably always be the minimum native ISO (not using any kind of ISO expansion). For landscape, that will probably be lower ISO's, such as 100 or 200. For action photography, including sports, wildlife, birds, kids, etc., the lowest ISO may change for every shot, and may vary from ISO 200 through ISO 3200 or beyond, and it will be very dependent upon available light. Lots of light will allow you to use lower ISO, less light will dictate higher ISO. Regardless of what ISO you use for action shots, another good rule of thumb is:

> Its always better to actually get a
> shot than miss one because you don't
> like the camera settings necessary to
> capture it.

Even if you have to use ISO 3200 to get decent indoor sports shots with your f/1.4 lens, at least you'll get the shots. Those ISO 3200 shots ***will** have **lower** noise than* the ISO 1600 shots that you underexpose then correct via post processing, as shown by the (rather extreme) example above. De-noising algorithms are also pretty advanced these days, and can greatly reduce the noise level of a high ISO shot to more acceptable levels. That again makes using a higher ISO that will ensure a correct exposure a better option than a lower ISO that will likely underexpose and require post-process correction.

  [1]: https://i.sstatic.net/rVqx6.jpg