when growing from seed, a lot of people top right above the second leaf set after the third set has formed. This gives you four main colas instead of one.
I myself use clones, so I wait till theres at least 6 growth points/ branches. Then top and leave 4 branches. This produces a similar effect as what I said of the seeds.
I've topped later but i've always gotten a lot of small buds (great for scrog). I don't do scrog so i like growing those 4 branches into nice big colas :)
Here's a pic of the clones befor topping and the other pics 5 days after the top. the 4 branches left over get nice and big and space between nodes gets real tight.
Some plants shouldn't be topped at all, depends on the plant and your growing style... obviously if you are doing a scrog you should top, and train, if you are growing tress, maybe not......
it all depends on what your lookin to get from your plant, atm ive got a 1x1m tent and top all 16 of my plants to create 4 tops per plant(64 tops in total) im also just finishing off my other room and i am lookin at 9 tops per plant
first pic is a 9 header
second is 4 main tops
1 thing ive noticed with topping is if you do it and nodes start to grow from these nodes they take up lots of space without yeilding as much, what im tryin to say is top when you have to but try to make sure each top grows from the main stem not a node from a node, hope this help a lil bro