I almost didn't tip in a cab once. I got into a cab and everything was great ... then the driver ran two red lights (not lights that just switched -- he went from a dead stop while it was red) and damn near hit a bike driver. I made a comment about it and he grunted and kept going. While most likely nothing would've happened, I decided to tip him simply because I didn't know what would happen if I decided not to; I'm sure the stories I've heard are overblown, but...