I agree with DebbieB. My DW and I use a limo in Chicago to get to/from O'Hare. I don't know what part of the fare/fee the driver gets, if any. However, a tip is optional based on service. If, like "nluvwithdisney" says, they show up late, the tip goes down. If they are really late, or slow, or lazy, then I don't feel obligated to tip.
If a tip is automatically included in the fare, then I usually do not add any extra, unless the service was superior and/or exceeded expectations.
TIPS = To Insure Prompt Service