How big is the quilt?
If it is too big, you may want to add a row of different fabric somewhere in the middle to give it that extra length. The reason is that for big quilts, the process of quilting will draw the backing and batting up a couple of inches & if your backing fits perfectly before you start quilting, you may find it ends up being smaller than your top once the quilting is done. If it is a wall hanging or something smaller, I wouldn't worry about it and I would just flip the fabric around to fit perfectly.