when I took my first quilting class 20 years ago the instructor recommended to always pre-wash and dry the fabrics. You can eliminate the bleeding of dyes and fabrics can shrink at different amounts, if one shrinks quite a bit more than a fabric next to it in the quilt, that will put added stress to the fabric's threads and may pull so much on the fabric that it may tear. If you are going to put work into a quilt why not eliminate any problem you can before it is a problem, and back then most were still hand quilting so it was a good investment of time. And btw calicos back then were equally 42 or 44 ", and some may have been 40" wide.
I use Shout color catchers when I wash a load of new fabrics. I bought them when they first came out because my son would not sort anything when he washed his clothes. It was much easier to buy the color catchers then do his laundry for him.
I had heard of using a cup of white vinegar in rinse water to help set dyes in fabric, I'm going to try the vinegar and salt treatment next time I have a tough one. I did get a link from Craftybear for buying the Retayne on line but I think it was from someplace east of me here in Ohio-so there may be several places one can order it. Closer may be less expensive in shipping, but maybe not, many online companies seem to charge shipping based on total order amount.
And speaking of Crafybear if you ever want a link for anything you saw on the QB she is AMAZING at getting the links, she is the Queen of the links!
And for the
small pieces in kits or such you can wash in a basin, and "shrink" by drying with an iron. No fraying of the cut piece and if it shrinks it will probably be workable for the block. If it is going to shrink don't you want to know before you have the whole thing together?