I always use color catcher in the washer with cold water when I finish any quilt. Have never had a problem, but if you have a lot of older fabrics (especially red or purple) where there may be an excessive amount of dye, then just test those in a cup of hot water. I make a lot of scrap quilts and do not prewash or pretest, but if its got a lot of deep colored scraps I put in two or three color catchers when its done.