How I experienced it was that seedlings grow better under 18/6, wether it's under floro, TL, MH or HPS and hortiluxes. I think they need that dark period to remain healthy at a top level, grow better roots and maybe photosynthesise better, but that's a wild guess since i don't posses that knowledge.
But clones on the other hand... They seem to do better under 24/7, especially with an MH or lower charge lamp. 24/7 under a HPS light is a bit too much for vegetation, in all cases, in my humble opinion, a 24/7 lower charge is good for clones.
But yeah. turtle's right, just adjust settings when you're growing