I don't. Uber seems to be trying to reset the clock on tipping. I've had drivers tell me that you don't tip in Uber. I'd like to keep it that way, so that it doesn't become yet another tipping service. I like that I can decide not to carry cash, can get out on the other side of the trip without having to deal with cash or any other payment systems.
Not having to deal with that stuff is half the reason I use Uber. If the drivers feel they need more money, that's something they should take up with Uber, not with the consumer.