I did not prewash my fabrics for years, then I started working with flannels which shrink in crazy ways. I also normally wash batiks, but I have a large stash and not all of it is washed. I just started a scrappy project and thought I had prewashed the scraps. Well, I starched a piece of red and it turned the ironing board cover red so I started testing the fabrics by wetting them, putting them on a paper towel with a glass of water on top and wound up throwing all the pieces in the wash. One of the batiks started leeching color before I put it on the paper towel and another turned my fingers pink. Most of the problems were batik, but a couple of blues that were Moda fabrics bled as well. I have found that if I want to be lazy and not prewash, I need to test the fabric first.