I am completely against giving the finger to drivers. It makes us look as a bad as them. That and the one time I did, I spend the rest of the ride wondering if I really set them off and would they be waiting around the next bend in the road. Turns out they were pulled over further on my route. They were out of the car and one of them made a lunge at me as I rode by. Luckily their car was busy giving another car a jump and they couldn't give chase.

I always smile and wave. It makes me feel better knowing I'm doing it in sarcasm, but it gives them no excuse to escalate the encounter.