I've actually seen bucks sparring and heard the occassional grunt on opening day here (Sept 15th).

Having said that, I don't think you elicit a positive response to antler rattling much before mid-October at the earliest. I'll carry the grunt (and bleat) call, but only to use in an attempt to turn a buck that's staying out of range. Late in Oct and early to mid -November are best for rattling and grunt calls.