For the most part if you purchase a quality fabric at a quilt shop it will shrink less than what you might purchase at JoAnn's or WalMart (yes there is a difference in quality). To wash or not to wash is usually a matter of personal preference. One word of caution though, if you are prewashing to make sure a fabric does not bleed, some darker colors, (especially reds), continue to bleed even after pre-washing. If I test a swatch and a fabric bleeds I treat the fabric with Retayne rather than pre-washing.