or you could always just tie it down. but anyway, best time depends on your goal doing so. If its a mother plant your doing it to control height and make it branch out more so i'd say after 5 true nodes feel free to top it. I would do a variation called fimming though(stands for fuck i missed, because the person who first really got it popular fucked up while trying to normally top it and cut it in the *wrong* area.) I'll get you a link if you'd like.