It's personal preference. I personally don't prewash, I did when I first started quilting 3 years ago, but I got tired real fast of dealing with all the strings. I always steam iron all my fabrics. If I am not sure about a fabric, my washer has a steam setting that I use then throw it in the dryer. I do wash all my finished quilts with color catchers...I haven't had a problem yet. I have a red and white quilt that needs to get washed before I give it away, so I think I might run use two. Just do what works for you.