Emirates is really clear about the fact that a tip to the driver is purely optional, and not necessary. They consider it to be somewhat like tipping an air hostess and the CD is considered a part of the "Emirates Experience".

Nonetheless, I would say that tipping should depend on whether you are delighted with the service, including the flight because its for the overall experience.

The amount of the tip should be based on what is customary for that country/city. In London, if you want to, I think 10 quid should do it. In NYC $15-20 at the most.

If you're not happy, make sure you don't tip and create a false impression that it's good service.