If you are folding the back over to the front, I would starch the whole back stiff...and be really careful how you handle the edges while quilting it... mine got wavy from grabbing onto the sides. I roll the edges and grip the rolls while quilting in between them.. If they still have a little wave, I would put the quilt on a table, and lightly spray the back all around the edges with water and hold the iron slightly above it and see if it will shrink back into shape.
You may want to try sewing one row of quilting lines one direction and then come back up the opposite direction instead of going round and round the edges in the same direction, too. That will distort the edges and make them wavy too.