Quilting pretty much keeps fabric from shrinking a lot if you don't prewash. I took a class from Harriet Hargrave, and she showed us a flannel quilt she had made to prove her point. She did not prewash the flannel and machine quilted the quilt. There was no shrinkage at all that we could see. She said that machine quilting keeps everything in place.
I don't prewash fabrics or batting because I like the gentle crinkling effect I get when I wash the finished quilt. I think this shrinkage and crinkling is primarily from the batting (I use 100% cotton).
The fabrics all do need to be similar in weight and quality, I think. For example, HH did not mix flannel and regular cottons in the quilt she showed us.