I always prewash not only for bleeding but shrinkage and chemicals with the chemicals being the big issue as there is a lot of formaldyde in the fabric along with lots of other stuff and I don't want to be breathing that or have it on my skin I have also found that unwashed fabric really dulls the sewing machine needles faster I got some fat quarters at my favorite quilt shop washed by hand and was amazed at the dirt in them so maybe just me but I am a pre washer