Originally posted by maltfalc wrong. increasing iso boosts the signal before it's digitized (except for extended iso), meaning less noise than adjusting exposure in software.
First, boosting ISO via analog amplification is only done at low ISO levels where noise just isn't a major concern anyhow. Who really cares about the difference in noise between ISO 100 and ISO 140? And once you get to a point where you do care about noise - say, ISO 1600 (specifics vary by model) - all ISO boosting in camera is digital anyhow. So there is *no difference* at high ISO settings.
Second, while there might be some theoeretical advantage to analog amplification over digital, there is pretty much zero chance that you would ever be able to *see* the difference except perhaps in the most extreme cases - eg, an ISO 100 image pushed four stops (digital amplification) versus one shot at ISO 1600 (probably mostly analog application on most camera models).
The point being, unless we are talking doing about something really stupid like shooting four stops underexposed at low ISO then pushing in software, the types of differences we are actually talking about here - typically ETTR might result in what, 0.3 - 1.0 EV higher exposure? - *just will not matter in the real world*. Even pixel peeping, it will be incredibly unlikely anyone would be able to tell the difference between a half stop push in camera or in PP. Which is to say, the actual benefit is next to nil. And then you have to balance that against the very real risk of blowing highlights.
Again, here I am talking ETTR via pushing ISO. That is what I am saying is pointless, and indeed counterproductive given the risk. If you instead perform it by slowing the shutter or opening the aperture, only then can you actually gain the theoretical advantage of this approach, as yes, it is indeed possible to see a half stop difference in noise, especially at higher ISO levels. But again, at those higher ISO levels, it's pretty unlikely you have the luxury of slowing the shutter or opening the aperture - you wouldn't be at such high ISO levels if you weren't already hitting maximum aperture and minimum shutter speed for your shooting situation.
So, I'd reduce the whole concept of ETTR to this: don't be stupid and shoot more than a stop underexposed or more than a stop higher ISO than necessary if you can help it. All else is just not worth worrying about, and does nothing but confuse people and risk ruining pictures.