Years ago I attended a workshop given by Harriet Hargrave. She showed us a quilt she had made out of flannel fabrics to prove to her quilt shop customers that it was not necessary to prewash because of shrinkage. She measured the quilt before washing and after washing. The shrinkage was exactly the amount expected from the batting used (about 3% for Hobbs 80/20). The same flannel fabrics washed on their own would have varied in how much they shrank, up to 15% for some. The reason the quilt came out okay is because HH had done her usual moderate amount of machine quilting before washing. Machine quilting binds the three layers together so they become "one" with shrinkage controlled by the batting.
Not sure how many of Harriet Hargrave's quilt shop customers were convinced, but I certainly was! This is why I do not worry about fabric shrinkage in the quilts I make (which all get at least a moderate amount of machine quilting).
Since I do not prewash fabrics, I typically also do not use spray starch on them. (The trigger on the sprayer is not good for my hands anyway, what with the beginnings of arthritis in my thumb joints.) I haven't yet run into a fabric that shrank from using steam in my iron, although I have heard of it. Guess in that case I would just use a dry iron.