I have tried both topping and not and I really prefer pruning at weeks 4 and 6 of an 8 week veg. The first time, you "top" the plant to bring 2nd tier branches up to the top. Do this by cutting the taller branches just above a pair of nodes about 1 - 1 1/2 inches above the lower branches' top. The second cutting is to prune lateral branches that shade their neighbors too much. It should go without saying that you make clones of these cuttings at least from one session.
Until you "max" the plant, each time you cut a branch it will grow 2 (or more!) tops to replace the one you took. These new colas won't be as big as the original but will be close. your yield of prime top bud will increase. There is, however, a point of diminishing returns. The top buds will be smaller, so if you top too much they can start looking like lower branch stuff. Experiment with your strain and see what it will tolerate. My HP13 will take about 10 prunings without adverse affect; they just get bushy.
As to pruning a plant whose sex is unknown: yes, prune it. When you clone the cuttings, do so with a 12/12 photoperiod and you will know the plants' sex. You can then cull (and smoke) the males giving your ladies more space to thrive.
I hope this helped .