When I was doing 135 it happened quite frequently, and I graciously and politely accepted every time. The biggest tip I ever got was $400 (each, 2 crew king air). When I was flying single pilot (chieftain), I'd often get around $100/trip. It was nice! We were usually tipped at least $50 (each).

Now I'm part 91, and we don't get tipped. But we're salaried and pretty well taken care of. Actually it wouldn't even feel appropriate in the situation I'm in now.