I prefer wide backs since once you get them straight there seems be less chance for problems. I don't have a dead bar on my frame so never use a vertical seam--otherwise I definitely battle sags and bags--so will load a quilt on the "side" (horizonally) if the back is pieced. I don't mind a pieced back, but have run into a problem on 2 customers that gave me backs that were pieced with larger pieces with a mix of wide and lenght of grain (and one was definitely very biased) that had some strange stretch.