That minimum tip idea makes a lot of sense, especially if you're taking short trips. In general, though, I don't apply the same rule to taxis as to waiters because the fare really is (more or less) proportional to the amount of work done, and it seems reasonable to make the tip proportional too. Whereas a waiter bringing you foie gras isn't necessarily doing any more (or more skilful) work than one bringing you a burger and chips.