Eh, it largely depends on my mood, or what effect I'm going for.
For example, by the lens pretty much means, "I'm intentionally shooting for intellectual jerkness," as well as a passing greeting. I also do it when focusing.
By the bridge is just a habit, as well as quick acknowledgement. As on the page, I also use it to flip people off. They never notice.
My glasses never slip, though. So I always adjust out of expression. Or after I move them down to read, or take them off for cleaning.