I generally tip if they handle my bag. Rental and Employee van drivers rarely do, but if they do they get $1. A hotel driver who doesn't even get out of the van does not get a tip. I'm a big guy, and don't really need help with my bag but it's the principle of the thing.
I always give the cabbies who pick me up at home a big tip...the word gets out and this ensures that when I call a cab to go to the airport I never have to wait for long. The dispatchers know me by caller ID.