I dont usually prewash unless its a color that I think will run or I am making a quilt that has several colors that arent close in color as in beige with green etc. I always prewash the colors that tend to bleed like reds, blacks, dark blues. I even wash the precuts in those colors by just placing them in a sink and letting them soak for a few minutes then rinsing until water comes clear. The only exception to this is when I am making a commissioned piece or something that I know I am going to sell. I then prewash all fabrics regardless of color and use Retain on them to ensure the customers have no problems with their items when they wash them.