Well,I'm a longarmer and I try to mount the longest sides of the backing on the frame,so I don't have to roll as often.So,I would want you to run the seams top to bottom of your quilt.If the seams are perpinduclar to the bars they stack up on themselves The rest of the backing sags and I get folds in it.That being said,figure out how you are going to have it quilted and make the back like the quilter requests.Not all longarmers mount the long sides of the quilt.Some mount the top and bottom.Just ask,I'm sure they can help you decide.