It is the history of the quilt and wouldn't bother me to have the shadowing. We know it is vintage. I would repair as little as is needed due to fraying and stabilizing and then enjoy them. You can find vintage fabric on line that may be similar to the age of your quilts and do repairs with that. Sometimes I have split a seam and added a piece of stabilizer to an area that is bad and I think it just adds to the flavor of a loved quilt. Repairs are part of the whole concept of quilting. Post some Pics. I would love to see them.