Normal lurker piping in here. My rule? It depends.
My last hair cut cost $90. I was not about to tip 15% on top of that, especially since the guy kept calling me by the wrong name.
Seriously, it depends. If he blows my hair dry straight and has an assistant shampoo and help straighten, I tip the assistant $5 and tip the stylist another $15. If he's feeling lazy that day and wet cuts it, then shoos me out the door, it's $10 for him and $5 for the person who shampooed me. But 20%? On a $90 tab? No.
There's another place I frequent which has folks who are doing their apprenticing, right after getting their license. They charge up to $40. I have no problem leaving 20% for them.