How'd Bud? What strain. may i ask?

Off the top off my head you can top them now. I assume they have a full set off leaves now?. Ive read some strains perform better than others when toped but you can top early on. Ive Topped my kali this grow aldo i wait'd till they were about 12 inchs, i wish i had have done it earlyer because Kali is a Pure sativa so they have grown about another foot roo foot and half since then. If your plants are Indica dom you wont have to worry about topping yet as said it'll come down to weather they are Sativa's or Indicas if they are Sativas top them before they reach 12 inch's

Hope this sheds a bit more light

Peace D