The rule I follow is never, ever touch a car.

As for confrontations, I usually don't do more than mutter to myself. If a driver's actions are bad enough (i.e. dangerous), I may yell, "Nice move" or something along those lines. I want them to realize what they've done or at least to know that I don't appreciate it. I can't say if it's ever made a difference.

And generally speaking, since I'm out there every work day on the same route, I really don't want to make enemies of others.