As a professional longarm quilter I know that tension can be a problem for all of us. However, I would never give a quilt back to a customer knowing the tension was off when I was quilting. I don't like to frog a quilt, but that is what I've had to do (2 borders of a queen-sized quilt) because they have to pass MY standards before I'll give them to a customer. Perhaps this quilter's standards are lower than yours, so continuing to go to her will only result in more disappointment for you.
Before I got a longarm I had another longarm quilter quilted my first quilt...she was using metallic thread on part of it and the tension was awful...she apologized when she returned the quilt and did not charge me for the quilting. It was a good example for me - if the quilting is crappy, I frog and re-quilt or give it back at no charge. I choose to frog and figure out my tension problems...I don't want crappy quilting out there with my name on it!!!