Yes, if I overtip, usually barbers too. Used to be, until a few years ago, that my haircut cost 60 baht, and me tipping 10 baht, meaning 1/6th.
I still usually tip 10 baht, but at 150 baht now, it is less than 10 percent. (Same barber, though, so over time it worked out for him.) Not keen to raise it to 20 baht, and a 15-baht-tip would probably result in being called 'cheap' ('Why didn't he tip a twenty?') behind my back, so, for now it is still 10 baht. - Sometimes I overtip taxi drivers, rounding up with more than 10 percent, either because they are friendly, or me not bothering to wait for them to search for coins. (Exceptions for taxi drivers from the airport who refuse to use the meter; they get no tip.)