I have honestly never had a problem with fabrics bleeding, but others have. I use predominantly batiks. Recently, I went back to prewashing with the color catchers. I washed 37 fat quarters by Barbara Brackman for Moda (Civil War collection) with two color catchers and colorless detergent, and the color catchers came out as white as when they went in. I wish I hadn't bothered, I ruined the lovely smooth hand of the fabric and spent an hour snipping off the thread boogers. I still have to iron it all. The only time that I prewashed and the color catchers did NOT come out white was when I used blue detergent. Just my experience, and I'm not that prolific in producing quilts as other board members. I just started using some old, bought on sale jelly rolls this year, and I find the Bali Pops to be perfectly cut as far as I can discern, compared to others which can be wavy, though likely no worse than I would cut.