Since I use flannel a lot I know it shrinks a lot. When I buy it, I buy 1/2 yard extra and sometimes that is barely enough. Many people say the quilting stops a lot of the shrinkage, I don't know how. And I buy quality flannel and it still shrinks quite a bit. So you could piece the top, fold over the edges like a hem, baste it down, and then wash it and dry it. But I would more than likely just wash the pieces I plan to use on a delicate setting using hot water, and dry them.