yah, i agree with smoking guns.. my last grow of two thseeds mk ultras, i didn't top them and i think my yeild suffered for it.. instead of getting two or four main colas depending on where you top it, i got one main cola and a few other decent sized small side colas and alot of smaller popcorn buds.. the nice thing about topping is that it makes the most out of the light that you have by making the canopy more even and therefore the cola's all recieve equal lighting.. i took four clones of the mk and are in flower right now and i topped them following uncle bens topping technique ( excellent thread with great info) and now that they are about a month in, i already think that my yeild will be much better as i have more main colas that are closer to the lighting..
i also have to herijuana seedlings in veg now that i am planning on topping as well..
i suggest the same thread that sg did, uncle ben went above and beyond on this one and it has tons of great info on it and is worth a peruse if your thinking of topping..