I prewash all fabrics to remove the odors as well as to take care of the shrinkage. I find that all of the fabrics I purchase from LQS shrink just like those I get at JoAnn.
I measure all fabric upon getting it home and then again after prewashing and drying in the dryer. Shrinkage ranges from one to three inches.
Kono black shrank the least. Robert Koffman and Beneartex shrank the most. Other brands range inbetween. My DH zig zags the edges for me before I wash them and it really cuts down on the tangles.