If you do top, be sure that your plants are very well supported. Where ever you top, a weak joint will be created that will be prone to breakage. This can be extremely problematic if the plant was topped early and a huge amount of weight is resting on that joint.
Personally I never do it to my outdoor anymore. I've done quite a few side by side tests topping the same clones next to each other in my garden. There was never any difference in yield on any of them and they all grew into nice round bushes regardless if they were topped or not. Topping increasing yield is an old wives tail in my opinion. Topping did however create weak branches, which took more supports, and created more work and more hassle. Training the main leader and branches as well as pruning works far better to control your canopy and does not have the negative effects of topping. If it is your first time growing I wouldn't bother with it, you won't gain anything and it is likely to create more problems for you later in the season.