It depends on the strain and what you want to do. If you grow big bud/chronic/critical etc topping is a bad idea. If you grow Herer, blueberry, blue mystic, NL#5 etc topping makes a lot of sense. If want a controlled canopy or have few plants, top and LST. LST is great for Sativa doms because they are much more bendable than most Indicas and grow tall. I can grow just about any strain in the same grow area by knowing the way the plants grow. For instance, one chronic I pinch the stem to control growth since topping is bad and LSTing defeats the purpose of the desired huge main cola, the others I top, the SSH I top and LST, a Thai pure haze I LST by bending the whole thing it at 90 degrees and have to wait two entire grow periods to harvest. Topping makes it much much easier to LST. Some plants Bush out on their own, especially after a good long veg in this instance it's great to do nothing or LST if it gets too tall.
It's also hugely dependant on the light being used. Say you're growing NL#5 I can almost guaranty that if you are growing one or two plants under a 400 watt it's better to top and lst because of the bad light penetration, making the canopy more even over say a 3x4 foot area, with better results concerning yeild. With 600/1000 watters light penetration is sufficient to grow large natural plants with obvious great success.
Now if you are talking about the difference in yeild over just topping a plant and letting it grow naturally most plant arent going to show much difference in yield and many would yield more naturally, unless it's lineage is like blueberry, or blueberry pheno dominant. Now if you veg 3 weeks, top, veg for another 3 weeks and lst it's a safe bet that this would yield much more than natural depending on the strain.
