I piece my backing all the time. The only time I consider what way the seam goes is if I plan to longarm it. If the seam goes side to side it's easier for me to keep the tension even. If the seam goes top to bottom, it tightens up the quilt in the middle where the bulk is and leaves the sides floppy. (as it rolls up on the rollers) But, if I'm tabletop quilting it , then anything goes.