Since everyone seems to fold quilts right down the middle, I figure I'd reinforce it on that backing seam. I sew 1/2 inch seam allowance, press it open and then stitch on each side of the seam about 1/4 inch from the seam. I match the fabric, so it's not too easy to see. Since I longarm it myself, I prefer the bulk on that seam to be open. They are charity or baby quilts, so I'm hoping they will be well used. So, reinforcing that seam just seems right to me. And, yes...horizontal seam........it's too hard for me to roll those guys and prevent puckers, etc. when the seam is vertical.