You should tip on the type of service the driver provides you. There are drivers that have you meet them downstairs, ask you to load the car, have nothing to offer on the drive and than expect a 20 % tips. They deserve nothing. Than there are companies that go all out in providing you with great service and information. You could give them what you feel. Anywhere from 10% on up. If you get a company that requests a 20% gratuity than check a few others first. They are sometimes the companies that have the worst service because their drivers don't have to work for the tip.