I prewash all 100% cotton fabric before using it. I use warm water and very very little detergent. If the colors are to bleed I want them to bleed in wash and rinse water instead of when washing the finished product! I dry the fabric using a medium heat. I want the fabric to be treated as if it were in the final product so all the bleeding and shrinking is done before hand. Sometimes I just use the rinse cycle, but I always dry the fabric in the dryer. This applies to 100% cotton quilting fabric which I usually get in a quilt store. One time I had some Walmart fabric...white and blue with a bit of red and that red bled and bled and bled.