Jeanne, right now I too am making a flannel quilt. The fabrics are top quality flannels. Here is my experience with them:
In answer to your question about whether the flannel would shrink evenly: no, it will not. Each piece will shrink differently along the warp and weft directions of the weave. (Regular cottons do this too) For instance, the precut 5" charm squares shrunk to 4 5/8 by 4 7/8 inches. There is a 1/4 inch difference in the length and the width of the squares now. Some of the colors shrunk even more. Within the same brand name collection of fabric, each color shrinks differently. That would have really horsed up my piecing if it was not washed at least once first. Flannel will shrink a bit again with the second washing, although not as much. I agree with the others, you should wash your fabrics at least once and perhaps twice.
I'm a die hard fabric pre-washer after having a finished quilt shrink up all wonky.