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.