I think its a bit both subjective and based on your needs.
I have heard doing 12/12 from seed gives smaller plants, but most I've seen is either 18/6 or 24/0. A lot of people say their plants respond well to 18/6 so I assume that's because the plants are stretching for light.
You'll need someone with a lot more experience than me to chime in however, sorry I couldn't be of much help.
As a side note to whomever helps this gentleman out, any ideas or experiences with simulating sunrise and sunset? I was thinking, just as a test (I doubt I'll yield anything different) to turn on and off sets of lights at different times to simulate dusk and dawn.
But, I've also thought of doing a slow-rolling light ring around the plants to fully simulate sun-rise and sunset. Thought being the key word. Its a topic that interests me for no reason other than to see what happens.
That should probably be in a different thread, sorry for hijacking PlainWrek.
Edit: As for power savings go, I'd move to CFL for vegging and use the MH lights for 12/12 at flowering. CFLs seem to be the most efficient as far as power goes, they also reduce the need for cooling and such. Vegging seems to be a lot more forgiving than flowering.