Grunt calls work great from pre rut on through the rut. I'm not sure about post rut as I'm usually tagged out before then.
Here in NE Indiana, mid-October through mid-November is a very hot time for a grunt tube. I like to rattle and grunt together at times. Other times, when a buck doesn't seem to be heading my way, a few short grunts will often times get him turned and coming in to me.
IMO, if you wait until the rut when they are on does, grunting won't work well because they have other things on their mind. Pre-rut seems to be the perfect time around here for calling.