I just messed up and kept them on 12/12 from last grow so I just went from maybe 2 weeks under cfl to a metal halide 12/12. I didn't change to HID for flowering yet. lol. It depends. Think of it like how it is outside. If it gets light out at 6 and gets dark around 8, then that is the natural hours a plant would get in the wild. So then as the season progresses, the days get shorter by a bit until it is only 12 hours a light of day. So you can play with the schedule like that. But I just switched the timer on flowering day and let it do it's thing. But I don't know if there is a science behind it, you know.
most folks just go straight to 12/12 with no issues. At the end of the day there must be some level of stress at this juncture as the sun doesn't act that way in nature. And plants do have a circadian rhythm and are photoperiod sensitive but who really knows. To mimic the sun cycle it would take months to get from 18 to 12 on a creep and most growers dont want to wait that long. Got to turn n burn those crops...................Sure would be a cool experiment tho.