but why the would you want to?????
topping the plant your gong to loose a week or so of veg time... and if your running straight 12/12, the plant is going t veg for prolly 3-4 weeks untill it is mature enough to flower, regardless of light cycle/photoperiod. im assuming you want this plant for bud, and not for the joy of growing;, so if you top it, you will end up with less bud.
topping plants is meant to be done when you have a long veg cycle, not straight 12/12. straight 12/12 is for SoG.