I've played with light times/timing quite a bit and here's what I have observed...24/7 can be a little expensive when using HID lighting, with CFL and Flouros, it makes very little difference in my electric bill at all no matter how much I run them, so I run them 24/7, this is for veggin. If you start at 24/7 and then work down on the light side half hourly or hourly until you get to 12/12, you just wasted a week or more imo, unless you've got a particularly sensitive strain. I grow AK47 and it doesn't seem to care much. If you go with 18/6 or 20/4 and switch for the duration of veggin, it doesn't matter...in veg mode the plants are generally more forgiving to light time changes. In flower mode they are not so forgiving, so get to 12/12 from whatever as fast as possible, I change bulbs and times overnight and go straight to 12/12 for two weeks to insure they get the message to flower and then go to 13/11 until done. I would not recommend noobs jumping, (and try to never go backwards!) around your light times during flowering, but in veg mode, it doesn't seem as critical...hope this helps!

An hour or less here and there doesn't seem to matter in either mode, (I have forgotten to close doors, or whatever to screw up a light cycle myself before) but make sure the dark time in flowering is truly dark and uninterrupted! Think about the outdoors grower and his plants...they see cloudy days, rainy days, bright sunny days, and moonless or moonlit nights....everything still happens, veggin, flowering and budding, but it takes the plant longer to do it. In the interest of efficiency (or speed of growth, which we assume is as fast as possible), I would not be changing things back and forth too much. The less stress, the faster the success with MJ, as far as lighting goes and too much change, makes too much stress, slowing the whole thing down.

IMO, the time for experimenting with light times is not in the middle of a grow, but planned from the beginning. Pick a number and stick with it as much as possible throughout the grow...you'll only slow things down changing things up in the middle of a cycle, (unless ADDING more lights) and you may not realize any significant saving on your electric bill either, and you may not be able to tell if what you change makes any difference or not, and then won't you feel silly because you did all these changes and you won't know whether you made any difference or not, or saved anything or not?

Personally I don't think you'll save more than a few cents between the two times you speak of with anything except high wattage HIDs (MH or HPS).