Unless you just want to reduce the overall size of the plant by triming side and lower branches. I would only top my plants at two weeks into veg and leave them alone after that. If you top them too late in the cycle they will not have time to recover and give new growth.
If you prune 2 or 3 weeks into flowering you will only reduce your overall yield. If your triming to reduce size and bushyness then you can prune anytime, but again if you prune during flowering yield will be lessoned.
That is my humble opinion.
Good luck