I agree with the prewash for a couple reasons. Of course it lets you test for color fastness. I never had heard of salt to set the color, white vinegar would be another method. I usually just wash it in warm with soap like any other load. Sorted by color of course! But other than color fastness, shrinkage in any sewing project can mess up the finished project.
That is why I prefer warm, or even hot water for whites and lights. whatever the fabric is going to do in the first wash it will do in pieces the same as in bulk. After the sewing if you prefer the sizing look, you can replace that! just use a little starch from the laundry care section at yor local store. I would probably use liquid rather than aerosol due to flaking. :) . I myself have never tried one of those color trapping additives so can't comment but, salt or vinegar (I suddenly want potatoe chips! :lol: ) are so much cheaper. And environmentally friendly!