That's a myth, that they need a dark period during the vegetative phase of growth. I've done the experiment to see for myself (which I recommend to anyone so you can see for yourself): two exactly identical veg boxes, with exactly identical lighting and ventilation and in the same room, growing clones from the same plant. After a month of vegging, the 24/0 plants were significantly larger and more lush than the 18/6 plants, which wasn't really surprising since the 24/0 plants were getting a larger dose of energy each day. That's what the Cannabis Grow Bible said would happen, but since I'd seen so many people say 18/6 was better (generally without explaining WHY it's better), I had to see for myself.

Both sets of plants were healthy at the end though, so 18/6 didn't hurt them or anything. They just weren't as big or lush, and it took another week or so to catch them up to the 24/0 plants.