Some people use the width of the fabric as a center panel and narrower strips on the sides to split the seams out. They say having a seam down the center puts stress on the quilt. I've never had a problem with that myself and I've made many with center seams. What I tend to do now is to add a strip of extra blocks or coordinating fabric between the backing pieces. Sometimes I use up what's left from the front, sometimes it's different, but goes well with the front and binding. There's another method (John Flynn?) where you cut the fabric on the diagonal and slide one piece down until it makes more of a square than a rectangle. The calculations gave me a headache, so I haven't tried it yet.
It's your quilt, do whatever you want! The only time it would really be an issue is if it's for a show. I've never done a show quilt, so I don't know what they prefer. I suppose it could vary from show to show.