what is the best time to top the plant?

it is better to top the plant when she have 7 or 8 nodes and before the pre-flowers show, or just top when they show the pre-flowers? all of this is do in vegetative stage, that i know but when is the best time to top?
p.s. thanks for all the answers.

All I would say is to make sure that your plant has been been in true vegetative (time since the first true leaves formed) for at least 4 weeks, and has at least 4-5 nodes. Plants can waiver on sex around the 3-4 weeks mark and any stress before/around this time is likely to cause problems.

I would recommend experimenting if you have a few plants of the same strain; try one at 4 weeks, one at 6 or so, one at 8, etc. This way, you will know what works best for your particular setup and strain.

thanks gaines, i have some strains and few hundred seeds, i try it with all strains and just wait to see whats the best strains to top.
i watch dr.grenn grow dvd and he top the plants in first day when he start to force flower, before he just have them 2 months in vegetative stage and plants already show pre-flowers and in the end he have plants with(each one) two fat big colas and many lateral branches full of buds, what you think of this method of topping?

Topping in flowering is a very bipolar topic. I personally like topping in vegetative because since it is still in the 'grow' phase, you get much more growth once it splits. I know several people who top right when they start to flower; this method does split into 2 colas as well, but they don't grow as large because the plant does not have as much time for growing.

It really depends on why you want to top in the first place. Do you want to grow short, bushier plants? Top them in vegetative if so because they will concentrate on lateral growth rather than growing a single stalk. If you want a tall plant with multiple, shorter colas on top, top them in late vegetative or early flowering.

Really, I would just play around with them... different strokes for different folks as they say.

Never top once flowering is induced. It can only decrease your yield.