Depends. If it's an annoyingly mandatory shuttle for logistics reasons (dfw offsite rental cars, for example, or a shuttle between airport terminals, or a iad mobile lounge ;p) I typically do not tip unless they helped me with luggage at my request.

For shuttles to hotels or offsite rental car lots, I typically do tip a buck or two... I chose to use the shuttle in that case rather than being forced into it, and these offer additional opportunities for marginal good service (like taking me directly to my car) that i am glad to express my appreciation for.

Particularly, I always tip extremely generously for the parking spot shuttle drivers at the LAX Sepulveda location. Anyone else with me?