I don't think many people tip, so anything is a bonus. We used to leave a couple cold beers in the cooler for our Smith river shuttles, but as the companies have changed we stopped that. I left them $20 on a $120 dollar shuttle this year and my rig was spotless when I picked it up. I don't think they really expect it, so a little goes along ways. I sure hope I don't have to start tipping 20% on shuttles, that might be a little high on most trips.
That's the first cash tip I've ever left... I doubt I'll do it on day trips, but long trips and nasty roads - it might just get folks to treat your shit with respect vs rallying over wash boards with the trailer whipping around in the back.
"If you dont do it this year, you will be one year older when you do"