When I haven't had any luggage I've tipped the driver around 50% of the time.
We've had drivers who have provided friendly, professional, personal service that goes beyond just driving the motorcoach. I'm happy to reward such pleasant service.
We had one driver who blabbed incessantly (including untrue trivia), played annoying music that's not part the Disney service, and pointed out that a sign at the front of the bus says that passengers could tip for exceptional service and that he had provided exceptional service to us. He didn't get a tip from me.
If a driver stores your bags from the luggage hold, I agree that $1-$2 per bag is customary when your bags are returned to you, with the amount determined by the quality of the service. I would consider the overall experience, not just the physical handling of the bags.