Since you are growing outside and need to keep them short, topping them or training (best option with autos) would both work to keep them below fence height, but I would advise against defoliating, especially during flowering, as this is exactly when they need maximum energy to grow their biggest and best flowers.
Just remember, -
Less leaves = less energy being made = less flower growth.
Actually, snce they are autos, you shouldn't have any problems keeping them below fence height, they rarely grow that tall outdoors, so I would be inclined to just let them do their thing with minimal interference.
Hope this helps, Organoman.