I always prewash. I don't want to take that chance of having my fabrics bleed into each other. I tend to buy a lot of reds and blacks so I believe prewashing is a must. At least once a month somebody on here posts their personal quilting horror story of how they worked so hard on a quilt only to have the reds bleed into the whites, which turned the white fabric pink, because they didn't prewash their fabrics. Then half the people who post either remind them to prewash to take out the excess dye or use Retayne to color lock their fabrics. If prewashing helps avoid ruining whatever I worked so hard then I'll take that extra step.