I purchased a lot of fabric this winter - measured everything before washing - also serged the raw edges - washed in hot (whatever that was at the laundromat) - dried on permanent press setting.
Then measured after drying and pressing.
Almost every piece had some shrinkage - quite a few of the pieces shrank at least two inches in width. This was BOTH quilt shop fabric and walmart fabric.
The Michael Miller black I bought shrank over two inches in width.
I want to know how the fabric will behave after it has been washed. If it has truly bad manners, I will either try to return it (sometimes it works!) or discard it. I don't want someone else's work to get messed up, either.
I consider prewashing fabric to be in the same category as properly preparing walls before painting them.