Keep in mind that Disney claims that tipping is included in the luggage delivery service... that is why they say you don't have to be in your room.
I agree that if you have the driver handle your bags then a tip is in order....... now, if the driver just gets off the bus and holds his hand out or they schlep for tips all the way without doing anything for it besides driving the bus, then a tip probably isn't in order.....
Tipping is for good service and I will do as such.....