I don't have a rule, but I do have a suggestion. I recently took a quilt with a pieced panel in the back to a long arm quilter. I had intended that the panel be placed about a third of the way in but I didn't tell her so she put it in the centre of the quilt back, and she probably worked very hard to get it that way. So if you are going to take your quilt to a long arm quilter, be sure to tell her if you have any special instructions for your pieced backing.