It is definitely better to load a quilt with the backing seams horizontal on the longarm, if that is possible. It doesn't necessarily require more fabric, only that you be more creative when you're piecing the back or when you're loading the top. In many cases you can load the top sideways to avoid that vertical seam. I piece lots of backs with left-over fabrics from the top, and since there are many seams on the back it doesn't create that single vertical seam that creates a problem for the frame. I haven't tried John Flynn's method, but I don't like the idea of a diagonal seam because of the extra stretch on the bias.