>>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.
Add a 3rd reason. To remove all the nasty chemicals used in the manufacturing process rather than touching/breathing them.