I use a 1/2" seam and reduce the stitch length a bit. I've opened seams and pressed to the side and neither have had an issue over the many years I've been quilting. Your longarmer may have a preference but mine does not seem to have a problem with either method. I like to have horizontal seams whenever possible but every now and again vertical seams work better yardage wise and I go that route. I love pieced backs and find them much more interesting than a one piece backing.