I tip 20%, BUT...I get it cut at an Aveda school so the actual cut is only $20-25. It does take longer (I can easily be there for an hour), but I've always been happy with the results. And I have long hair, which I figure is lower risk :)
ETA: If you have short hair, any reason you can't DIY the color at home? If you are doing highlights, you can get one of those little caps with the holes to pull chunks of hair through, a bit tedious but works much better than DIY foils (foils are definitely best left to pros). If you are doing a solid color, my mother always swears by getting two adjacent colors and mixing them to get a color that doesn't look like it came from a box (you can use 1/2 of each for one coloring and then save the other 1/2 for the next session).