I tip just the 18-20%...regardless of luggage. That's part of the job. I figure that if I have a lot of luggage, that tip is now well earned, but I'm certainly not tipping anything addtl for it.
I tip 20% but not any extra for handling luggage. You are paying more for the limo service and thus end up paying a higher tip than you would pay a taxi driver (even if you still tipped the taxi driver 20%)