This is a good rule for interacting with tipped US workers in general: be generous, and tip in cash. Tips are a way for companies to save money by avoiding taxes, and a way for workers to be paid without their employers taking a cut. If you think Uber is paying your driver less than minimum wage, make up the difference in cash. Also, "internet points" in the form of stars cost you nothing, so give your driver five stars unless he was completely incompetent.