You could top now, or a couple weeks before switching your light cycle. If you have a magnifying glass keep an eye out at the middle internodes for preflowers. << They look like this if they are female.
Check out Ub's sticky on topping. Its in advanced. Will answer all your questions and more. In general best to top when you have 3- 5 nodes on your plant. Then cut about the 2nd for 4 main colas. Hope it helps Good luck