I prewash. Because
I had $9.99/yd quilter's quality fabric run horribly and (thankfully) ruin a table runner (thankfully because it wasn't a full-size quilt and the screaming and yelling from me would have knocked the Earth off its' axis). I got a full refund on all the fabrics I used in the tablerunner and the shop removed the offending bolt off their shelves and returned it to the manufacturer. So now everything gets a good prewash before I cut it up.
I don't prewash for shrinkage, because I like the antique-y puckery look. So I don't prewash/dry my batt, either. I do like to spread it out for a day or so to relax the folds, tho.
Yes, prewashing does remove the sizing from the fabric, taking away the stiffness of the fabric. I've used regular starch when pressing my washed fabric to add back that stiff hand to help with piecing if I think I need it. Generally speaking, I don't mess with it much.
If you're worried about shrinkage/stress on stitches/dye running, you can always hand wash (UGH!) in your tub in cold water. You can also add a dye fixative to the water to keep the colours from running. Also, when you add your label, make sure you write/stitch/print washing instructions, like DO NOT MACHINE WASH, in very large letters.