I like to do it after the 4th node on a seed. clones may be slightly different because they will have alternating nodes but the same idea basically. there is some good info for people new to this here. the pic below is from the link below.
Fimming is similar to topping but you don't remove the whole top down to the stem.
With seedlings that haven't reached sexual maturity, topping when you have 4-6 nodes is good for making a dual main stem(or even making a quad iff you top each of the duals as soon as they get a couple nodes.
But if you let the plant get about 5-7 nodes and then FIM, you will cause the lower branches that are developing at each node to pop out and race to be the top main cola. This will produce a plant that looks like a candelabra. This is an ideal method for plants that are going to be scrogged as you create multiple fast growing branches to fill in a screen
Either one would work fine. It is more a matter of which methods you are using. You could top it initially when it is about 3-5 nodes then FIM it when it gets up to about 8 nodes so that it will bush out more.
You can't do this with just any plants though. Some Indica and Indica dominant plants don't take well to topping or ffimming. Make sure you check with the breeders or ask about it on the forums before doing it.
Actually you can fim about the same time you top. I like fimming because I am not cutting off a bunch of top growth and instead of 2 tops you usually end up with 4. I just think it is less stressful to the plant.
Hush brought up a very good point. Some strains are known for their large almost single cola plants--these do not do well with topping or fimming. What strain are you growing?
the strains I have were sold to me by the names afgani, master kush, girl scout cookies. I hope they're ok to fimm, cause I did. I'm told the strains all have taste's of there own. i wanna taste that Colombian gold dirt taste. and smell that dirt smell. what ever happened to that strain. smoking that would be a trip back to high school for me.