SS: CDock feather in provides a little more clearance and will straighten out some arrow flight problems. However, if the nock point is set right and the arrow well tuned to the bow, it isn't necessary to turn cock feather in. A lot of folks do, though, because of the better flight one gets if everything isn't in tune.
I switched to 4-fletch several years ago, but when I shot 3-fletch, after I had my arrows flying like I wanted them, I never paid attention to how I nocked them so I might be shooting cock feather in on one arrow, and cock feather out on the next. They all shot the same.