I have AAA and it has come in handy with my not-so-great running cars over the years. I've never tipped the tow truck driver, then again, I've never paid them either; its part of my membership. I would say if they had to do something out of the ordinary, yes, I might tip, but if they are simply jumping my car, changing my flat tire, or hooking me up to tow, then no.
The only time I did, and this wasn't a tow truck I had called, was when I left my car in a parking lot as the weather was crappy, and it wasn't plowed, and snowed over, and I coudln't get out. My dad was helping me, and a tow truck drove by, saw us, came and pulled me out. No charge, but my dad and I insisted he take somehting. I think we gave home $20 or $30 (this was 20 years ago)