i don't usually prewash fabrics except if I am unsure of the quality of fabric. Some brands shrink just a little, others a lot. Also, It is totally normal that you will loss some inches when you are done quilting it, also the first time you wash it. I have kept a notebook listing the size of my quilt before quilting, after quilting, and again after the first washing. I also list the type of batting and thread used. This way, I can reference back to how much shrinkage to expect and will make queen a king sized quilts bigger so I end up with a quilt of the desired size. Also flannel seems to shrink more then regular cotton but again, there is a large difference in flannels too. For example Moda flannel does not shrink nearly as much as the bargin flannel I picked up a JoAnn's. So when working with flannels, I will wash them first if I do not know where they came from. Unless of course I am making a rag quilt, then I don't because unwashed flannel frays better after you wash the finished quilt.