I've rarely had to ask for changes, and the changes I made were very minor. The artist had no problem with the changes either. I learned to trust their advice and I have never been disappointed. @Pugilist provides a very good checklist in a recent post. Also, kindness, courtesy, and respect will go a long way, as both @HaydenRose and @irezumi noted. These are permanent designs, so it makes sense that we should be comfortable with what we get done. At the same time, people should take the time to find out how the process works and what the customs are in whatever shop you choose.