whats a good tip?

Hey guys I have a few tats, and my buddy moved that did my ouline on my leg sleeve, now I need it shaded in and i want to do some bio-mechanical where all the open skin is. now just for the shading lets say they charge $100. I will be going to a shop and having it done, whats the right amount to tip, i know some people dont tip and some do, is it proper or expected to get a tip everytime? i know if I like the way it came out I will return and give him more business, so whats the rule of thumb on that? it seems like your already paying an arm and a leg at a shop anyways. thanksguys

hard to say, all depends on the finished work, but i would think that if you plan on going back again would be a good tip. also you are going to tell your friends about the place too.