Best not to top, LST is the way to go, just tie down the main stem and/or branches too.
With topping, this causes a "recovery period" for the plant, and generally none to very little growth during this recovery time. Due to the limited veg period of autos before flowering begins, any delays caused by topping eats into the veg growth phase, meaning smaller plants when flowering commences at around 3-5 weeks of age, which obviously then reduces the flowering potential and outright yield.
Basically -- topping = delayed growth while recovering from topping = smaller plants during veg = less yield when flowering.
Stick with 18/6 and avoid defoliation. You want the plants to grow as quickly and as fast as possible during veg and before flowering starts. The more leaves you leave on, the more energy your plants can make, which leads to maximum growth. Your autos need "sleep" to function properly, and 18/6 is the ideal mix between of being "awake" and growing and "sleeping" and having a healthy metabolism.
Hope this helps, Organoman.
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