I used to never prewash. I've started because I'm using more of my own hand-dyes. I have to prewash the fabrics I dye to get the sizing out, and then wash them multiple times to rinse of excess dyes. I'm worried if I mix them with commercial fabrics that aren't prewashed the shrinking won't be the same.
But if I use soley commercial fabrics, I pretty much never prewash.