I would prefer not to see tipping become an expectation. That said, I won't tip unless there's a way to do it from the app. The whole beauty of this thing is not having to deal with cash, and I refuse to return to that. Additionally, it becomes problematic for expense reimbursements when I have a credit card charge that shows the fare, but I'm then trying to explain that there's a separate expense for the tip (which I don't have a receipt for).