I tapped a cabbie once when he slowed suddenly in front of me to park on-street without signalling.

The one time I got right-hooked I had to use my left hand to keep my balance as I cornered with the car, and then I was too shaken up to respond with anything more than an incredulous shake of the head.

I don't carry anything heavy that could cause damage. I figure a light tap is OK if you don't think they've seen you.