I have always wondered the 18/6 vs. 24/0 arguement, so last year I did a test, just for fun.

I had some bagseeds lying around from previous good bags. A freind of mine had just bought all this equipt. and shit, his grow teacher gets arrested, so he was left with all this lighting and shit and no one to teach him how to use it, grow, or have any seeds, so I helped him out as much as I could. So we decided to plant 20 seeds total, 10 seeds would veg under 24/0 cycle, 10 would veg under a 18/6 cycle, then thrown into flowering. Now, before I go on, I was told some time ago by a friend, that less light will produce more females than males, that's how this whole test came about. The results were good.

*10 seeds vegged under 18/6, we got 7 out of 10 female plants.
*10 seeds vegged under 24/0, we got only 3 out of 10 females.

That's what we did, and those are the reuslts, all written down next to me as I type. Hope this helps somewhat.