I prewash. I put new fabric in the laundry room after I walk in the door. I have had fabric bleed but my main reason is allergies. I received 59 blocks from members of my guild. I suppose some are unwashed, or come from houses with smokers or pets. Regardless every time I work on putting those blocks together, I get the sniffles and a headache. Sadly these blocks have set for over a year and I need to deal with them and finish putting the top together. Once I quilt it I will be able to wash it.