Like the other posters, I've always tipped 20% too since that's what was suggested by their websites. I also tipped half of it on each leg of the trip in case that I didn't have the same driver.
For a $200 fare, I would tip $20 way....unless I had really bad service. But, that doesn't happen all that often. Going rate is 20% of the fare...so if you break it down in half, that's going to be $20 each way.