I follow local custom. There are countries where tipping is either unusual or only for extraordinary service. Not so in the States. It's functionally part of the service person's wage. Like it or not, you pay for the service one way or the other. There's no such thing as a free shuttle. It's just a built in cost associated with the room. If the hotel paid more and charged more, it wouldn't really matter.

I also tip very well to key people with whom I deal on a regular basis, such as the evening doorman at a hotel I stay at all the time. When I need a cab, I know one will materialize while others fume