I take car services multiple times per week, at home or on the road. Over the years, I've used various providers in the NY/CT area, as well as Boston Coach and Carey when on the road, including abroad. In every single instance the car service costs billed to my employer already include a tip, broken out in an itemized accounting of costs.

So, it's no different than when a restaurant adds an automatic 15% (or 18%, or 20%, or whatever) service for large parties, or your hotel room service bill has a required % service charge, or when a resort does the same to your room bill. There is no expectation or need for additional tip, that is the point of those charges. So if you're one of those people that still likes to tip extra in those situations, then do so. If you're not, then don't tip the driver any extra either. They are getting their tip already.