Please don't feel like you are over thinking the backing. Everyone has gone through this. You can piece your backng either vertically or horizontally. The only thing that I watch out for is that the backing is pieced consistantly one way or the other. The only time I've had a problem is if a section of the back is horizontal and the other section(s) are vertical. Sometimes that can cause a saggy spot in the back. And I usually make the backing and the batting a few inches bigger than the top. The quilting process can cause a little shifting, so the extra backing and batting give a little insurance incase that happens.