You can prune an MG any time from the top or anywhere along the vine. All it will do is cause the vine to shoot stems off below where you pruned the vine.

From my experience, it depends on when you prune the vine down as to how much it will leaf out and bloom more.
In otherwords, if you have a long enough growing season, and you do it now, it will fill out more and bloom more.

Since our weather has been so crazy all spring and summer, only time would tell if you get a bushier vine and more blooms.

Either way, though, you will not harm the vine.

Fran