Should be fine to give it a good chop. You'd usually do it in May time straight after flowering but i've cut mine back in August before

I use the shears on mine as soon as the flowers go over and get a new flush of blooms later on.

Agree it should be cut back immediately after flowering - but, given where you live, whether you can do yours now or not depends on when your first expected frost date is - if its early (like September or October), I wouldn't recommend anything other than a light trim this late in the year.