I always prewash. I like to see how a fabric will look when it is washed and how it holds up in the wash. I have had a few surprises along the way!
Fabrics are also treated with many chemicals, not just dyes. Those chemicals are terrible for you to breathe in and work with. Recently, I have had some "good" fabric that I washed that had the most awful smell AFTER I washed it. After about 10 washings, the chemicals and smell still wouldn't come out. If I had not washed it first, I never would have known.