I admit, I do it all the time. I practically have to. But I normally don't do it to actually find a key (until recently when starting to learn a new layout); in QWERTY I have long known exactly where they all are. I just do it to make sure my hands (and fingers) are actually where I think they are. It really does seem to significantly reduce the chance of mistakes and trips up to Backspace.
In fact, something funny I noticed lately: I am learning the Dvorak layout and have Dvorak-labeled stickers covering the keys. The stickers cover the entire key including the QWERTY labels, with the only two not covered being the "A" and "M" keys (because they are the same, of course). I can still type practically error-free in QWERTY by looking down occasionally to "recalibrate" or "sync" my hand-eye coordination. Meanwhile, if I don't do it, my rate of typos skyrockets.
I was taught to "never look at the keys!" in typing class (never mind the fact that they had black fingernail polish on all of the keycaps... LOL), and any time I read an article or forum post on touch-typing, they always look down on it as being "wrong" or "bad." Yet, even when learning a new layout, it seems mostly beneficial to me. Maybe I'm just a visual learner...