I have tested 18/6 vs 20/4 vs 24/0 and I did not perceive any noticeable differences in growth rate. I now run 18/6 because it uses the least amount of energy and therefore is most efficient.
There is no need to go to 18/6 before going to 12/12 though. I also don't believe in 36 hours of darkness to "induce flowering". Flowering is a funny thing (again, the way I look at it, I'm not a freakin' biologist). Every plant has a genetic clock that tells it how long the flowering period is going last. If I only have 9 weeks of flowering lets say...I want my plants to receive as much light as possible in that period to give me maximum yield and potency. I don't want shortened days, I don't want days when the plants were unhappy/stressed/stunted/hungry/thirsty/pissed off at their girlfriends, whatever. I want the plants to have 60-70 days of pure happiness and bliss. Every day that's not is lost potential. This philosophy has worked for me over the years and I don't mess with it anymore.
As much light as they can handle and as much happiness as I can provide, and they pay you back in dividends. Don't cut off lights at the beginning or end, it's lost hours of light that the plant was using for production.
I am open to other insights, but after years and years of hearing "theories" about the gains by trying all of these "tricks", I just go about doin' my business the normal way and I'm happy with it.