I prewash. I worry about diff fabrics shrinking at diff sizes. I worry about bleeding. I think it's just easier to wash them in advance.
I put them in a lingerie bag, throw in the washer on delicate/cold cycle. Then put them in the dryer on "warm". I take the time to go untwist them every 10 min or so.
But I am not an experinced quilter, a newbie. I just worry IF the fabric should bleed or fade or ravel, I'd rather be prepared to deal with that. Instead of making a whole quilt and one piece of fabric mess up the whole thing!