If not waving makes you a dick, I guess I'm a dick about a third if the time. Sometimes I don't feel like waving first, and if there's a lot if bikes out, I get tired of it after a while. I almost always RETURN a wave, tho. I do like some of the thoughts expressed here... if you're on two wheels your in... that might be my friend when I break down.
It is entertaining to see the differences. Around the Minneapolis area here, there are a lot of low, pointing-at-the-ground waves. In Iowa, I've seen a lot of hand-in-the-air high waves - actually waving the hand back forth, like trying to get someone's attention in a crowd. In Missouri, EVERYBODY waved at me as if I was their neighbor. Sometimes get a helmet nod, and I've noticed mostly newer riders doing this. Maybe they haven't learned the local customs yet.
I always wondered too, if riders told their female passengers to handle all the waves. Usually with two people, I see the chicken the back wave.