The topping technique is mainly used for plants with opposing nodes. Im assuming right now you have alternating nodes, unless its been reveged for a good bit. If you still have alternating nodes, a better route may be to lst each branch outward away from the center. Do that for a few weeks and try to keep the canopy even. You can get several tops this way also.
You could top it anytime you want, as long as it has good roots going and is healthy overall. But it you have alternating nodes, it typically won't have the same effect as doing it to a vegging plant.