I've been doing some Uber driving recently during bar prep. So far I've received three tips, only one of which was from someone I actually did something for (carried heavy bag).

The one time I really think you should tip your driver is if you're hiring someone to take you on errands. I do this at least once a day: pick someone up from their hotel, take them to the bank or Fed Ex Office or to a grocery store and idle in the parking lot while they do what they need to do. As I get 10 cents a minute and make far more off mileage, 45 minutes of errands can net me a whopping $6. If you're hiring your driver for such activities, I think a tip is more than fair.