You can do either. Ppl , including myself like the 18/6 cycle because it saves on $$, and more importantly it duplicates "mother nature". Always a dark cycle in nature. But like I said, 24/0 will work.
You can put them on 18/6 now if you want. Just don't go changing the light cycles around after that. Pick one and stick with it. Plants that get stressed out by fucked up light cycles tend to go hermie.