I was in St. Paul, MN last week for work and Ubered to the airport. The driver and I were chatting and this came up. His take is that Uber considers "no tipping" to be a core part of their platform and discourage it. Drivers are supposed to say it's not necessary if passengers ask, which he does. Even so, he estimated 30% or so will tip anyway. He will accept it with thanks but is not upset at all if a passenger doesn't tip. He said a good rating is more important to him.
I brought it up, not him, and I did tip him $5 on an about $20 ride. And rated him 5*. He was nice.