For competition it is recommended that the back is 3 pieces rather than a seam in the middle. A lot of people now use pieced backs with extra quilt blocks added to make a quilt reversible. If it is going to the longarm quilter, I think they perfer the seams to be horizontal rather than vertical? I'm not sure about the last part but it has to do with rolling the quilt on a frame and the seam allowances making a lump. Do what you think is best for it's intended purpose.