Despite the seemingly representative anecdote in your last link, I'm very hesitant to believe that most Uber drivers in San Francisco expect a cash tip. This he

Despite the seemingly representative anecdote in your last link, I'm very hesitant to believe that most Uber drivers in San Francisco expect a cash tip. This hesitance is based primarily on the fact that I have never heard of tipping with cash, and I know very few (if any) people here that routinely carry cash, especially small bills. But perhaps the practice really is more common and expected in other demographics, like tourists.