I agree with Daisy Dew. I came from a designer of clothes background. If you are going to wash the item after made , you need to pre wash the fabric. Even in clothing. As flannel is used it puts stress on the seams. Also good thread should be used. The seams will pop as the idem will wear at different rates. Also flannel will stretch a bit more than regular cotton. All that said I love the flannel quilts I made. BUT I have learned the seams need more than 1/4 inch seams. Do it right first time so I don't have to redo it.

I am not a washer of my cottons, but I learned the hard way to pre wash my flannels unless I'm making a flannel rug quilt. Flannel does shrink much MORE than cotton.

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.

I, too, use a half-inch seam on flannel. They are so soft and cuddly that they get LOTS of use, so are washed a lot--so, for safety's sake, I like the extra seam allowance--flannel does fray.

Flannel
 

I have made several flannel quilts. I don't wash it. I use color catch when its done , it must all shrink the same, and have never had a problem

I wash all flannel and use Retayne to set the dye where necessary, which is frequently.

I'm the odd ball in the group I don't wash any fabric I buy. I have made many flannel quilts & haven't had any problems. Also use flannel for backing on cotton top quilts. & never had a problem!!

After putting together a beautiful rag quilt for my niece I put it but washer to "rag" it. The material did not shrink but the colors ran. (I had one light block next to a dark block). All that work could have been saved if I just pre washed first!!

I would wash the flannel material, especially since each piece might shrink more than another, and might make your blocks wonky when the finished quilt is washed.

This seems to be the old to wash or not to wash controversy. For my own peace of mind (or is that piece of mind) I wash everything but most especially flannel. The most important with flannel, as far as I am concerned, is that .5 inch seam allowance. Whether not it shrinks it does fray.