We had Zoey done at 7 months...I would deffinately try and make sure it is before the first heat.
Zoey has never really humped, but I would be shocked if she did not only because she is a girl but because she has bad hips. She does do this thing with one of her boyfriends that is a dominance game that is basically her version of humping though. They are so weird - first he will hump her, then he will lay down and she will stand over top of him, then they repeat - it's like 'ok you're turn to be dominant, now my turn, now your turn, my turn'.