I never prewash my flannels before making quilts - I love the way they come out of the dryer, looking like they are the most comfortable quilts in the world. I made one for our church bazaar a little over a year ago and the woman who bought it absolutely LOVES it, when she was going through some health issues this past summer, she would curl up with it and said it made her feel like everything would turn out all right (which it did, thankfully!).
As long as both sides have not been washed, they will most likely 'adjust' equally, and you will have a lovely, comfy quilt for a very long time. And if you don't like the way it looks when all is said and done, I would be happy to give it a new home.