Ok, I'd let them grow until a week before flip. They'll have a bunch of stems then. At that point it can be time to take all the unwanted and unlit leaves, and branches that are stunted. Three weeks into flower I take another look and touch up. I like air flow in the plant.
That said, do you know when you want to flip the light? It's flexible with the plants growth in my garden. I grow them so many nodes high, (so they don't grow into the light) them top knowing the 2 to 3 week stretch (intense growth of puberty) is coming. LST ing along the way.