So I plan on going to a Supercuts for the first time later today. What should I expect to tip for a haircut around 15 dollars. I was unable to get an appt for my normal barber, who charges 12 for a cut, but I pay 15. I would like to give 20 each time because it is such a great barbershop, but I don't have the money for that.

I usually stay away from places like Supercuts because it is a gamble on whether I'll leave with a good cut. I just don't want to feel obligated to leave a tip if it's not an awesome haircut.