Not to interrupt or be too pushy or anything, but, last year at a local fair, my Sister in Law entered her quilt and took best of show. (Purple Ribbon) Her quilt back was pieced together with extra blocks to mimic the front in some places. So I guess that like others have said, it really is the discretion of the judges.
I have been to guild shows that have been open to the public where most all the quilts had different fabrics on the backs pieced together. In fact one quilt was "double sided". It was actually 2 quilts in one and it took the highest honors of the guild and peoples' choice.
Do what you think is best and in my opinion, may be pleasantly surprised. Good luck and keep us posted on how things go?