There's two schools of thought for placing seams on a longarm. If they're vertical (perpendicular to the rollers) some longarmers feel that they make a lump in that area as they are rolled, keeping the quilt from rolling evenly. I recently watched a video by another professional longarmer who said that this is not the case and that the seams should be kept perpendicular to the rollers, though I don't remember what the reasoning was, or who the longarmer was. I think it had to do with keeping the quilt perfectly square. At any rate, as far as I know it is just a longarmer's preference, and since the longarmer can usually place the quilt in either direction on the frame it really shouldn't matter.