I also prewash my fabrics. I make a lot of baby quilts and pillowcases for kids in hospitals and feel the finishing on fabrics could be harmful to them, plus prewashing takes care of the shrinkage and throwing in a Colorcatcher takes care of the excess dyes. I also like the flat finish look of quilts and use a good poly batting or when using 80/20 batting I presoak it to shrink it. I know most of my quilts will be tossed into the washer and dryer and the steps I take keeps the quilt looking flat, not puckered.
Funny aside, I made a quilt with 1930's repros and wanted it to look puckered so I didn't prewash fabrics and used all cotton batting. Wouldn't you know the quilt hardly shrunk at all, LOL! May in Jersey