I LST before they hit the screen to slow down the gangly ones, and will do the same once I'm training in the screen.
I've even done it upwards of 1 to 1.5 weeks into 12/12, but don't like doing it after that.
Personally I like to have the screen about 85-90% full before I switch the lighting schedule - I may veg a week or two more than some, but I like to have a fully involved canopy that fills my area with bud.