But as to the original question - I would hand wash it or wash it on gentle in a front-loading machine and line dry it. That should help minimize damage to the seam allowances. Then press it thoroughly with a steam iron to help complete any possible shrinkage. If it's still in good shape, I would quilt it.

I'm with Yvonne I never prewash and knock on wood never had a problem. I like working with the unwash fabric better and like the comfy look the quilt get after washing it