I sometimes wash, sometimes not, depending on the cuts I'll be doing. Sometimes the stiffness of the manufacturer's sizing really helps with cutting . I had some large pieces ( 3 yards plus each) to wash this week, and really wanted to minimize fraying, so I washed them in some king size cotton pillow cases closed with rubber bands. I put a color catcher in and it worked great! Washed beautifully, very minimal fraying, and got a couple nice clean pillow,cases as a bonus : ). My first quilt was for my granddaughter, in greens and purples and I did not prewash. When I washed it for her the first time, I used a couple color catchers and the quilt looked awesome...the shrinkage gave it a lovely aged look. Since then it has not shrunk any more that I can see despite several cold water washes, and she really loves it.