This is an interesting topic, and as a taxidermist I have tried to put this all together as well. I think for the most part. The main growth is all but done by now and the additional growth will be very minimal and for sure only occur at the base of the horn. However, I personally think given the right conditions, the horn will continue to grow "very slightly" until the rut.
That being said, this is what I've noticed. We know that their horns are made of hair and fingernail type material. We also know that they shed the sheath every year and start growing a new one immediately. I have noticed that bucks killed in August up until about Sept 10th or so, still have a lot of long hair at the base of the horn that is actually part of the horn and comes off with the sheath (on normal moisture years), indicating that the horn growth is just finishing up. Bucks killed at the end of September have little hair attached to the sheath at the base, and there is a slight ring indicating that the sheath is drying up and preparing to separate from the horn underneath (in my opinion). Bucks killed from the 10th of October to the end of October are starting to show signs of a loose horn sheath. Most bucks will drop the sheath by November 15th, some as early as the last part of October. I have removed horn sheaths from bucks killed as early as October 9th, and the typically hairy meat matter under the sheath actually has about 1" of new horn growing at the tip. in my opinion this buck would have shed his sheaths prior to the end of October. I have notice that almost all bucks killed after the 20th of October, have about 1" of new hard horn cap growing underneath the existing sheath.
Bottom line is this. In my opinion, a buck has the potential to keep growing horn up until the rut starts if the conditions are right. However, the growth from July on will be very minimal. Also, I think that a bucks horn is actually starting to dry up and shrink by mid October, as it prepares to come off.
sorry for the small book I just wrote.....