I always prewash. I used to do it to make sure I don't have shrinkage or bleeding later... But now there is another good reason to prewash fabrics... BUGS!!
Bugs of all kinds are becoming more and more of a problem.. and with the bed bug epidemic growing every day I never bring anything fabric past my laundry room.. which is where I come into my house from the garage. I not a bug-a-phobic, but I'm not going to give those little varments a chance around here!!