I always (or as much as possible) load my quilt back with the seam running across, not vertical. Vertical loading with a center seam is just asking for trouble in my experience. But as for the sagging, I think it has to do with how you load the quilt, not how the customer sews it (although I can't explain why it would be better in the other case). But maybe it's a case of things not being completely square? Try to picture loading a slightly wonky rectangle onto your frame. The side that is "longer" at the top is going to sag initially. Then, when you get near the bottom, the other side is now "longer" and will have extra fabric to sag.