Tipping a Limousine Driver

Limo Drivers: I have mixed feelings on this one. Limousine companies usually charge between 100-300 dollars per hour to rent. On one hand id say that you should tip the driver Very well, simply because they have been your personal little slave for the day and driving you around anywhere you want to go, and open the door for you, loading your bags, etc.
However, I think that there is also a side to this that you could say, well they make so much money anyways, the company gets a cut and the driver gets a cut. Tips are just a bonus, not a requirement. So I suppose it actually just depends on how you see the whole limo issue. But I do not know of any specific percentage that you should be expected to tip a limo driver, I think this is at your own discretion, on an individual rental basis.