I am going to get my hair cut by a good friend. We have a good relationship where we give each other stuff / buy each other things without worrying about who owes who what, so money has never been a great concern.However, should I still be tipping for the service? On the one hand, I would love to compensate them for giving me a haircut. On the other, I'm afraid it might be offensive if they are doing it out of friendship.