Most battings come with instructions on whether and how to pre wash and dry. A cotton batting will shrink more than a polyester one, and if you don't pre wash will give you that old fashioned look. I guess it just depends on what look you want. I am not aware of any disadvantage to not pre washing. I am usually so excited to get my top sandwiched that I forget to even think about pre washing! I have heard people say that you can just spray it good with water and iron with a hot iron and that will take care of some shrinkage, but I have never tried this!