Yeah, this makes me crazy, especially since it always seems to be the clueless project managers who touch my screen.

Personally, I have long arms, big hands, and terrible close-up vision, so I approach other people's monitors very slowly, with the back of my pinkie, and really do my best not to touch the part of the computer that isn't supposed to be touched. Anything else is just rude; you are sticking your finger into the retina of the person who owns the machine.