I don't top my plants any more. Some strains don't like it. I tie mine down. (a la LST)

The ones right under my light I LST to keep them short, while i let others grow tall and put them at the outside of my circle of pots to make like a stadium effect.

If you are going to top or prune your plants, I would suggest no later than 2 weeks before flower to give the new shoots some veg time to develop before flowering.

The best is to just experiment and find out what works best for you under your conditions.