I always prewash - especially strong colors like reds and dark blues and greens. I don't want bleeding after all the work of piecing a top! And if the fabric was a real deal, or a batik, for sure it goes in the wash first.
I just started buying Thimbleberries fabrics and they came with a note that they are made not to bleed, so prewashing is not necessary. Not even recommended!
I am going to leave them unwashed and see what happens.