Don't do it yet because it'll still be in flower until it gets affected by frost. Mine flower into November, easily.

Keep cutting the stems down to the ground when each flower stem is finished and new stems will keep coming right back.

Also, if you're uncertain of the cold and frost levels where you are give it a good mulching to see it over the winter months.

I would divide in the spring when the soil's going to be warmer for transplanting the divisions.