I only prewash when I have to - like for a block swap on this board-LOL. My most recent experience w/that reinforced my feeling that I don't need to prewash. The focus fabric in the block swap was black. I purchased two different black fabrics to use in the swap and was dreading the washing process. I loaded the washer with color-catchers and crossed my fingers. Neither of the black fabrics ( or any of the other colors) bled!!! I will happily continue to NOT WASH my fabrics until after my quilts are done!