It depends on what you want. Do you want all the same fabric on the back - if so, leave it in one piece.
The only time I piece 'same' fabric for the back is if I can't get a single piece in the whole size that I need.
I'll also piece different fabrics together either for a "pretty" back or to use up scrap.
If I had a whole piece that fit though, I'd just use it that way.