I grow mainly autos so not alot of topping for me, however i use LST for all my plants and i start as young as i can. As long as there is something to LST i do it. Probably around early week 3 like you, but a picture of your plants would help us determine that better.
When i do top, i also try to top early as possible aswell. Lets see some pictures so we can help you out.