I prewash fabric that has dark or intense color, and any that feel stiff. All the fabric in a quilt is either prewashed or not, I keep my stash sorted like that.

color fastness
 

I recently bought a small handbook on using salt, lemons, vinegar, and baking soda. They have a recipe for color-fasting your laundry (quilt fabrics or quilts). They use 1 cup vinegar, 1/2 cup salt, and your color-safe detergent (or quilt fabric soap). Filling the washing machine with cold water, add the three ingredients and wait a few minutes to let the salt dissolve. Wash your brights with similarly colored items in small loads and the colors will be set for future washings. It says to double the amount of vinegar and salt if colors are particularly vibrant.
I haven't personally used this recipe yet so if anyone beats me to the results please share.

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?

genhaven, what is "color safe detergent"? Is that different from regular liquid detergent such a Tide or Arm&hammer?

retayne is sold on amazon; joann's; ebay and probably some other sites just do a search for it.

also i have used white vinegar successfully to help set colors and this is from my garment sewing days. it does work.