The only time I'll ride the shuttle in LV is to and from the rental car center. Many times the shuttle driver will specifically target the carry-on's and leave the heavy luggage to the customers themselves to load. Offloading - they'll request all passengers to get out and wait for them to toss it to the curb and expect the tip.

Given the $15-20 in airport fees I pay on my rental, isn't that enough of a 'tip' already since the drivers obviously get a wage? If I get a friendly driver who I can tell has been working hard and geniuinly trying to help rather than fish for tips, I'll appreciate and tip a couple bucks. But in the last 3 years, I've only come across 1 driver who I felt deserved a tip (I always do carry-on and my bag is less than 25lbs)