The perfect exposure is the one that results in the perfect print.
So, first you have to decide what would constitute the perfect print. Then, you'll want to experiment with different ways of determining exposure until you find a workflow that lets you get that perfect print.
It might be that you normally only shoot well-lit low-contrast scenes in a controlled environment. In that case, your best bet is generally an exposure in which a gray card gets rendered as middle gray, or a traditional "correct" exposure.
It might be that you're shooting a high-contrast scene where you don't have any control over the light. In those cases, you've got some choices.
It might be a field with lots of dewdrops with colorful specular highlights that you want to draw attention to, in which case you want to expose for those highlights lest you blow them out and lose all the color. That might mean ETTL, or "expose to the left," which would be okay because you were planning on crushing the shadows anyway in order to provide more contrast with the sparkly bits. Nighttime cityscapes where it's the colored lights you're shooting are similar.
Or, it might be that your subject is dark and in shadow, that you really want to bring out its texture, and you're planning on blowing the highlights again to provide contrast. In this case, it's ETTR all the way.
You might be looking to do a bit of both, in which case you're either back at the "standard" gray card exposure as a least-worst compromise, or you need to use some other technique to tame the dynamic range (such as HDR or graduated filters, if not lighting / reflectors / whatever).
The one thing I'd recommend against is automatically getting on the ETTR bandwagon. The whole point of ETTR is to reduce shadow noise, and it does so by effectively reducing the ISO rating. As such, it only makes sense for low-contrast scenes shot at base ISO -- and modern DSLRs just don't have any appreciable noise in such settings. In high-contrast situations where you might wish for less noise in the shadows when you have to boost them in post, ETTR is going to blow out the highlights. That's great if you care more about the shadows than the highlights, but lousy if you care more about the highlights than the shadows. And, at anything other than base ISO, you're much better off lowering the ISO than using ETTR.
Also, most ETTR workflows apply the exposure compensation after the tone curve has been applied, which results in all sorts of weird tone and chroma shifts; if you're serious about ETTR, you really need to be working with the linear RAW file...something that's generally waaaaay more hassle than it's worth -- in no small measure because you're then giving up all the wonderful modern tools like Adobe Camera Raw and Lightroom and their shadow boost / highlight recovery / etc. stuff.
Cheers,
b&
