A friend that used to own a quilt shop (until she retired to quilt!) told me that there are two times to be sure to pre-wash--when it's a fabric that is likely to shrink very much (like flannel or cheap muslin) or when you take a sheet of clean, white paper and rub it hard on the fabric and it "shows" on the paper. Usually that will be with black or other very dark colors or reds--but even then she said that quality fabrics rarely bleed. So now any time I have a doubt, I get out the paper and rub away--some times I find I need to wash, others I don't. Despite ironing, it seems that the fabric is never as crisp once washed and that makes it hard to cut/sew. And I avoid using starch or stay press, etc if possible as I don't want bugs being attracted--plus then it feels "gluey" when I sew it. that being said--the first time I wash a quilt I always use a Colorfast sheet!