Normally I top a plant early in veg after 4 real sets of leaf nodes. I veg for 2 weeks and top again, or flower. You will want thick stems for big buds. Lot's of fan action builds thick stems. I have a tall screen right now and have been shoving 5 month old mother plants in there. Other people will SCROG into screen a bit lower, fill it up and train during veg early on. I've found the longer you veg your plant, and train it for even light distribution along the main bud sites, the more solid buds you will have. If you top wait 2 weeks to flip, let the rest catch up. I'm not sure what i'm going to do with this next bush .. I might rip the screen out, and rope it down like i was talking about previously.