I generally sew two WOF pieces of fabric together RST, after doing away with the selvedges and then taking one side, rip it down the center (ripping actually works better for me at this point). I have a little card that I got from AQS when I subscribed to their magazine that shows how much fabric to buy for each size quilt, and on the back it shows both vertical and horizontal piecing for the backing. I don't think it makes a lot of difference, except possibly the amount of fabric used, as long as the selvedge has been removed and you have a shorter stitch length when sewing the pieced back.