That's exactly why I pre-wash. Fabrics do not shrink at the same rates.
I think somewhere in the processing fabrics should get a nice hot bath but apparently they don't, so I pre-wash. Sometimes I even find a fabric or two that bleed, too, but it is the fear of uneven shrinking after I've finish making something that keeps me prewashing.
I do buy brand name quilting fabrics, usually on sale but if I need a color I might buy anything I can. I pre-wash!
Jois