Doesn't it depend on the batting you will use, also? Even if I've prewashed my fabrics (for fear of bleeding) there will be some 'shrinking' due to the cotton batting I've used. It's sometimes inevitable, don't you think?
I've been told NOT to pre-wash batting. I can't remember why..... just that it was not a good idea to do so. I think it might have something to do with the batting falling apart if you let it go through a top-loading machine (the agitator will pull it apart).