dmnf

If just ride in and out, I don't think they need tip.
If he/she comes out and open and close the door, then 5% or round up.
If I have luggage and he/she needs to put in/out the trunk, 10%.  

Some drivers are very rude, he drove off the cab before I close the door.  Those I won't tip them.  I may even complain to their company.