Almost all the rental van drivers help with my luggage and I always try to tip $5 if I can. I usually travel heavy and really appreciate the help getting all the bags, stroller, car seat, etc. in and out of the van.

If they just drive, then no.
Or sometimes I've only got a $20, then I may or may not ask for change... that is a bit awkward. But I try to catch them on the return.