I found this webpage that recommends 1/2 cup of vinegar in the rinse cycle for flannels. However, I have some doubts as to the reliability of this advice as some of it is wrong -- e.g., that vinegar will "set" colors and/or prevent bright colors from bleeding. This(used to be true for some organic dyes, but it does not work for modern chemical dyes.
http://bobbleroos.blogspot.com/2012/...illing-of.html