> When using your webcam, do you look at its lens or the screen?

It varies. If I'm listening to someone else I usually look at the screen, if I'm speaking I'm looking at the lens.

To echo what @StavrosK said, I don't think that looking at the screen when the webcam is above the screen would be perceived as inappropriate.

I don't think anyone finds you looking at the screen inappropriate because it looks like you're looking at their chest if it were real life. Maybe someone who's never seen a camera or screen.