Sometimes I think it is a personal preference to wash or not wash. It does wash out the sizing but I like to pre-wash my yardage I always use the Color Catchers when I wash also. Your can always iron your fabric after you wash it and use either Spray Sizing or Spray Starch to give it back a little of the crispness the fabric may have lost in the wash.
If you don't know about Color Catchers they are like dryer sheets but you use them in the wash and they attract the dye that would otherwise bleed onto any other fabric. They work pretty good.