For ever so long I always did the vertical seam and just dealt with the overage and additional scraps. Then on a HUGE quilt I went to a well known and very respected quilt shop and they calculated the amount of backing fabric I would need with both a horizontal and vertical seam.. Big difference in the amount of fabric and therefore cost. Since then I will often calculate the back both ways before making a purchase or cutting from existing stash fabric. A penny saved here is a penny for other fabric I may need later there. Right?!