So as I was fine tuning my RD after a recent install (see franken derailleur thread), it occurred to me that perhaps there is a "right way" to set the wing-- either "right" in the sense that it should be set 1/2 way to allow either increasing or decreasing tension or "right" in the sense that "its just the way it done"; sort of like how one's rim labels are supposed to face in the same direction (see very long thread on that issue not long ago).
Do cables ever shrink? Once set, why would you ever need to lengthen the cable?
I fiddled with those once, (on a 9-speed bike), I discovered that the wing allowed me to adjust the cable tension exactly 1 cog's worth. I'm thinking that's the max you should ever have to do on the fly.