I would say if s/he goes the extra mile or does something that's out of the norm, take care of them.

My instructor has gotten many a tip from me because he spends so much time with me. The average (around here at least) lesson lasts 30-35 minutes and costs $25. My instructor, Lamar Dean, if you happen to be in Tampa, sits with his students for at LEAST an hour for $25. I took approximately 15 lessons from him and my shortest lesson was an hour and a half. My longest? 3 hours. He charges $25 per lesson, regardless of length. I often gave him an extra 5-10 bucks. Sometimes it was just because I didn't have correct change, but most of the time it was because they guy cared enough about my progression to spend extra time with me making sure I understood what he was saying.