I think they have suggested to wash in case they bleed. If they are already warning of a bleed, I would test a section of each colour. Take a wad of about 8 inches and put it in hot water. Take it out of the hot water and put it onto a white paper towel/kleenex. When it's dry, check the towel for stains. If it bleeds, I wouldn't use it. I have never washed my floss but I have switched to the pearl cotton thread on the little ball for embroidery. I still pre-test a section even though it says colourfast on it.