I prewash my fabrics for the most part. It's not as much because of shrinkage, it's more because I like the feel and behavior of fabric that has been pre-washed then starched. There are a few exceptions:
- I had not been pre-washing cut fabric, anything smaller than a fat quarter, because of the unraveling. But, I just recently read about washing cut fabric pieces in a lingerie bag... so I bought one, just haven't tried it yet.
- If I am working with some pre-cut pieces that haven't been washed than I don't wash any yardage I'm putting with it.
- If I am working with a panel, I don't pre-wash anything - that messes up the panels. Learned that the hard way!
- I always pre-wash my flannel (and now starch it too, makes a big difference).
Hope this helps!