I don't prewash batting. For people who want a completely flat look (no batting shrinkage), it's a lot easier to buy a poly batt than to prewash a cotton batt. Some traditional cotton battings cannot be pre-washed; they will fall apart in water, or the surface bonding agent will dissolve. A cotton batting such as Warm and Natural can be prewashed because it is needlepunched through scrim.
I personally like the antique look for my quilts. Antique quilts were made using cotton batting, which shrinks just enough in the wash to give a softly krinkled look. Modern cotton batts shrink anywhere from 1 to 3%, which is just right for me.