I do my own quilts and do use a 1/2" seam when seaming the backing. I iron the seam open. I do the seam on the horizontal. I didn't think it would matter, and I did a seam on the vertical. It made sense to do it that way because I needed about 80" width for the backing and that way I only had to sew 1 seam. It was a big mistake. Every time I rolled the quilt on the Longarm frame, the center got higher and looser on each side of it. I fought that quilt to get it to stitch evenly. I now, either do a diagonal seam now or a horizontal one when I piece the backing.