I do longarming, and it usually doesn't matter, because whatever way the seam goes, I will load it so that the seam is horizontal, and work on the quilt sideways if need be. The only time that might be a problem is if the quilting is directional and the quilter will have a hard time creating the images while looking at them sideways, something which seldom happens, I'm sure. It's never happened to me yet. I often prefer to load a quilt sideways, because I have the table length, and that's just fewer times I have to roll to a new section.