topping is strain specific. If you have a plant like Kalimist that is a dominant cola type of producer, it's best to let it be. But if you have a plant that branches a lot( like sour diesel), it's most certainly going to increase yield if you top it and allow a few weeks in veg to recover and grow sturdy branches. I will top my plants at 8''-10''. Because I grow my veging plants to 24'' before I flip the lights to 12/12. An early top allows the plant to focus energy more on branching and less on top cola height. By the time my plants are 7 weeks from rooted clone, the branches are nearly 12'' long and are all the same height. I also supercrop the stalks that are the tallest (pinch and bend technique) up to the third week of flower. This allows the lower branches to recieve the hormone that is normally only delivered to the top.