I'll join the odd ball group. I very,very rarely prewash any of my fabrics. Only if there is a spot of some sort that I missed when buying. Don't iron anything until I'm ready to work with it. Only had a problem with 1 red flannel running otherwise all have worked out well.

After reading all the replys I am sure what ever you chose will be fine. My stash is so large that I am not sure what I bought from where. But after making the puppy rag quilt from Simplicity patterns I chose to wash the flannels as they moved into my home. I have made several of those puppies over the years and they are always a hit. They have to be washed before you give them away. That's when I learned to pre-wash flannel. I had to resew many seams.

I am not a pre-washer of fabric...but I do prewash my flannels. They have a much higher rate of shrinkage than normal cotton fabrics do.

I wash all my flannel and use 1/2 inch seam. It helps in the long run!

You say you bought a kit - are they precut squares/strips? If so, I wouldn't pre wash unless you hand wash.

I always wash all my fabrics before I use them, but with flannel, I do wash it twice in hot water and dry it in the dryer. It is amazing how much it shrinks...lots more than quilting cottons. Also, do not use the selvedges, cut them off! I do longarm quilting for others and never knew anyone to use the selvedges in their piecing and backings until recently...the weave is so much tighter that it deflects the needle when I quilt it and I hate to see what happens to the quilt when it is washed because they didn't pre-wash the flannel! With all the pesticides sprayed on the fabrics in the warehouses, I wash it all as it comes into my house...I don't want pesticides on my hands or in my lungs. I would not use a 1/4" seams either because of the way the flannel is woven, it is looser than cotton and will fray so much - a flannel quilt I did has had some separation in the seams, creating holes, so make sure that you use a full 1/4" or a little more for seams and quilt over both sides of every seam to help keep it all together. The more quilting on a quilt, the better it will hold together with use.