Hrmm, I've never really tipped a ref before in my time playing, but generally I am also just a walk-on with maybe one or two other people with me. If I were there in a private group, I would definately chip in a few bucks to tip a good ref though, probably even a bad ref because private parties tend to be full of people that are more difficult to work with(Not meaning that necessarily in a negative way).
But I usually appreciate it when I find a good ref that chills out and chats for a while during down time or offers pointers. I've been playing for years, but never steadily, so I appreciate tips on things. I've also been called crazy the first time I went to a field with my brother because we played a 1v1 while three refs watched and both sprinted for the center bunker and started blasting each other at about 3ft away.
But I will keep in mind from now on when I go to keep an eye out for outstanding refs who actually look like they enjoy being there and keep a 5 or 10 on me if I need it.