Generally I wash everything first. But with pre-cuts I don't bother. I also do a BOM at my LQS where they give you tiny bits at times...don't pre-wash those either. And with those, I end up also buying yardage for sashing/backing/whatever. In those cases I don't pre-wash and so far have been very lucky. I've also pre-washed and still had issues...ya just never know! I think it's the batting that really determines how much shrinkage you're going to get with your quilt as long as you are using consistently good-quality fabric. If you're mixing good and lesser quality fabrics I think that's where you'd run into more of an issue with varying shrinkage rates.