I've gotten a lot of questions from people about the appropriate amount to tip a cab driver. As a public service, I'll explain it here (from a cab driver's perspective, natch, so feel free to take it with a grain of salt).
My rules would be if the fare is under $6 and on the low end of the dollar, you're pretty much fine with completing the full dollar ($4.35, you can leave $5). If you're at the high end of the dollar, you should probably leave one extra full dollar ($4.75, leave $6). Once you get over $6, leaving $7 would be ok.....but probably better to leave $8.
After those low dollar amounts, the general rule would be to leave between 15% and 25% or so, like at a restaurant. And if it's a lot easier to leave a teensy bit under 15%, then that's ok (on $8.75, you can leave a $10, which is only 14.3%). We cab drivers generally do tend to have a lot of singles on us, though, so if you really do want to leave $11, we can get you four ones with out any trouble. (an aside: please - large bills can really be a pain in the ass. Last Saturday night, I took a guy from Shirlington to Georgetown, an $18.45 fare. He handed me a $100 bill and told me to keep $22, which is a perfectly good tip. But coming up with $78 in change isn't THAT easy, and it meant that after he left I had to go back to my apartment to get more cash so I could make change for future customers. That wasted a half hour of my time, which comes out to about $20 because it was a weekend night. That really sucked. I'm happy to make change, but I'm not a bank.)
Red Top takes 6% juice on credit cards and on vouchers, if your business happens to use those. So if a fare runs $15.95, and you leave $20, that is usually a very generous 25% tip. But if you use your card, the driver actually only keeps $18.80 of that - it's still a perfectly fine tip (17.6%), but it's not as generous as you thought you were being. There have definitely been a couple times where I've come away with less than the fare or just gotten a tiny tip because of the 6% juice. That Bethesda trip a couple nights ago for example - it was 21.95 on the meter, and he gave me 23.95 on the voucher slip. It was already only a 9.1% tip, but after the juice, I took home $22.51, which is a $0.56 tip (2.6%).
A couple last things to mention - there really are a lot of people that over-tip, so if you happen to only have enough cash on you for a 12% tip or something like that, it's no big deal. A driver's time is usually far more valuable, especially if it's a weekend night, than the coins you gather up to make sure you hit 15% instead of 13%.
And finally, and most importantly, a tip is actually a TIP. This whole post assumes that your driver was pleasant, efficient, and safe. If you aren't satisfied with the service, you have no obligation to tip at all, and you truly never have an obligation to tip as long as you can pay the full fare. But for the sake of karma.....