I go by the rule that if the driver doesnt fall asleep at the wheel then he gets a tip, and if I have to give directions to/from Arabian Ranches then they don't get the tip (but still usally give in as I know they are paid quite low), otherwise a nominal amount of 10-20AED usually in Dubai.

In Toronto it is $10 as with LAX, London GBP 5-10 depending.