I usually top my plants if they are over 6 feet because then they start to wilt over. It will also force root growth, and leaf growth. I topped a plant about 3 weeks ago, and it already has almost doubled it's root mass and has a full new set of leaves on it and is shooting right now. Two shoot since I topped it.

-Steve

I've never been a big fan of topping bamboo, and avoid doing it unless the plant is really in a bad way. I'm not convinced that topping causes significant rhizome growth that wouldn't occur naturally, and the plant looks butchered. I know that topping is supposed to bring the leaf mass back into balance with the rhizome mass of a new division, but I don't believe this is always necessary. I've made divisions without topping and have even had new shoots come up shortly thereafter. For me, the key is getting the biggest rootball I can.

To top or not to top?: that is a question on which reasonable people can disagree...