[ipsi]

I think Gett (a Black Cab app I use in London) improves upon tipping somewhat as well - your choices are 0%, 5%, 10%, or 12%. That's it. No giving a 20% tip (or a 100% tip because you were drunk-typing or whatever).

I've always just set it to the maximum tip amount, because I figured (a) It's not that much, and (b) it would result in me getting better ratings, though after what GP said, maybe they have no idea how much I tipped :-/