Here are my thoughts regarding hotel/airport shuttles:

If the trip is from the airport to the hotel and it is part of a regular run with other people, then I generally do not tip. After all, the driver did nothing extra just for me.

On the other hand, I have stayed at several hotels where I made an appointment and the driver drove me either to the office or another location with just me in the van. In that case, I felt a trip of $2-3 was appropriate.

Once on a business trip in Florida, we had the hotel shuttle drop the three of us us off at a local bar/restaurant and he offered to pick us up again in two hours at a specified time. In this case, we appreciated his offer, saved the money on a cab and gave him $5 when he returned to the hotel.