I longarm my own quilts. I did only one quilt with the seam vertical to the rollers. Even though I had cut off the selvedge edge and ironed the seam open, I still had a huge humpy by the end of the quilt and had to fight with the top to keep the very slack edges of the pieced top from having nips and tucks. That was the last vertical seam in a back for me. Now, if I can't use a single piece back, I buy 108 to 120 inch wide fabric when I see it on sale, I always make my seam as one that runs parallel to the rollers. I mostly use pantographs and they are almost always set up to run across the quilt and do look similar to rows even though the edges are inter woven. They don't look as good to me if they run lengthwise on the quilt top.