I just did some googling and one said that your fabric thread counts may differ, therefore, if you don't pre-wash shrinkage of the various fabrics will also differ. Also, if you use muslin for a backing which doesn't match the fabrics thread count, the muslin may shrink more. And, you can pre-wash the fabric but don't forget about the cotton batting because that will shrink, giving it the antique look.
I'm going to wash some fabric and batting and iron with starch. I'll make something simple then wash that, and see the difference. I just have to know. Funny thing, I never thought about pre-washing the batting...