I thought this was a pretty blatant cheap trick the first time I saw it (similar to restaurants putting tip calculations on bills from 18-25% (which are usually post-tax no less!!)) and on the occasion I am in a cab always punch in "Other," or best option pay cash. Automatic deduction for cabbies who often tip themselves and hand you less change without asking - give me my change in full and I'll tip you, thanks.
To add a data point on tipping amounts I would say I usually tip 10-15% for a good cab ride or less for a bad one, or even zero if the driver "screened" my fare (common from yellow cabs in Queens - keeping the doors locked and asking where you are going before letting you in, which is illegal) or takes unnecessary routes, blatantly excessive slowing to hit red lights, etc. I might venture into 20%+ for a good service ride to/from an airport.
Restaurants I stay around 15% for fair or better service, 20% for good, much less than 15% requires the server to be noticably rude or inattentive. Sometimes I do feel like an outlier or cheapskate as the "norm" service for anything feels like it has become 20%, which I find a little ridiculous. I won't not tip because I don't "believe" in the culture of tipping, because all I would be doing is hurt mostly decent workers; at the same time I won't default to a high tip for average service just because other people are doing it. I try to find a middle ground.