I do unless they are rude or I have luggage and they don't offer to help me with it. If they are inside the hotel talking with the staff and don't help me with my bags I don't.

I did a park and fly at a BW last year and I didn't tip either way. Going the shuttle was full and it was one of the smaller busses and the driver was inside talking with the hotel clerk and didn't offer to help anyone with their luggage. coming back they had two signs that said "tips appreciated" posted and it was just a van and the driver didn't help me with my bag that way either (large rolling duffel that weighed almost 50 lbs) so I didn't feel obligated to tip then. Had they helped me with the bag then yes I would ahve tipped.