I rarely prewash - only when I suspect that a fabric may run. Usually bright primary colors. I recently had a dark bright green solid bleed so badly despite hot water, several washings, vinegar etc. that I had to throw it away - and it came from my LQS. And I do prewash flannels because I don't trust shrinkage. Nevertheless, I prefer to work on unwashed fabric. We all have our preferences and quirks.