If you have unwashed or mixed fabrics in a project you can use the color catchers when you wash them-that should help keep colors from bleeding into each other if they bleed. The only problem then might be if you have an unwashed fabric that has a lot of shrinkage. I was taught too to prewash. Shrinkage variables being one reason and fabric bleeding the other. Many on here have stated they don't prewash and don't seem to have a problem. I have had some fabrics that with washing and rinsing still had color bleed after 2-3 times wash and double rinse, one was a couple of yards of a really nice tourquise color I may have to treat with retayne before using to get it to stop. I can imagine it would be very burdensome with out a washer/dryer.