I always pre-wash and mostly don't have a problem. However, I once washed a yellow fabric in the same washer as a red piece of fabric. I didn't take the fabric out of the washer right away and the red bled out on the yellow fabric and I never got it out - had to cut around it for quilting. However, the red never ran again, although it might not be fair to call it bleeding, but it sure stuck to the other fabric. When I have the time, I like to wash the fabric, toss it in the dryer for about half the time, then iron it, makes ironing go so much faster.