Here's the thing. Quilting prevents fabric from shrinking. Sewing it to the batting and backing stabilizes it. They can all shrink together, but one cannot shrink way more than another.
100% cotton batting, which is what antique quilts were made with, could not be prewashed. That is why antique quilts are crinkled; the batting usually shrinks about 3%. If you like the crinkled look, as I do, there is no need to prewash fabric for shrinkage.
In other words, fabric shrinks differently when it is washed by itself and when it is washed in a quilt that has already been quilted with reasonable closeness.
Starching *before* cutting instead of after takes care of keeping the pieces the correct cut sizes for piecing together.
In my opinion, the only reasons to prewash fabric are (1) to stabilize the dyes in a bleeder fabric, and (2) if you like a flat, modern look to your quilts. In the latter case, prewashing fabrics should be combined with a poly batting to prevent the usual 3% shrinkage of primarily cotton battings.