In my experience, both grunting and rattling works better EARLY...Once the rut is in full swing, the bucks are too busy dogging does to pay much attention to calling..
This may not be the case in other places with different buck/doe ratios, but in upstate NY it holds true...
Calling is worth trying ANYTIME during the season, but I have had much better luck doing it the first couple of weeks of bow season...