When to top an auto: NEVER. Autos rarely withstand the kind of stress topping does to a plant... they've got their own schedule and if you muck it up, they may not have time to recover and you could lose your whole grow... So put that right out of your mind unless you are a very experienced grower... and even then, put it out of your mind. LST is the most stress autos can deal with...
Your LST looks very good... just keep adjusting it, i.e., continue to LST as she grows... you want to get the bud sites well out away from the main stem and into their own light...
Good luck!