First off, let me make this clear; I am not stating that any perticular photoperiod is "best".
The photoperiod you choose to run for vegative growth is truly a personal choice, based on your unique set of circumstances.

In my personal grow, I have a seperate mom/veg room that uses 18/6. Durring the 6 hrs the lights are out, the exhaust from that room goes to the flower room (during lights on) and increases the ambient co2 level.
When I transplant clones that were rooted in RW macro plugs into 4" RW delta blocks, I use 24/0 from LED. Mostly because the shelf I do that on is located in my work area, and cuttings are also rooted there. I can get roots growing into the delta blocks and veg the plants till they are 6-8" tall in 10-14 days, then into flower.

@Rusty, I would post a scientific paper dealing with "no harm to plants" if I could get files from my old computer. The paper I'm referring to was done by Dr. Raphael Mechoulam (the grandfather of cannabinoid research) who had to back up from that research and do some basic plant physiology to show his colleagues and critics that his use of 24 hr vegative growth for many of his studies had no effect on cannabis grown this way. BTW I think you'll find that the same chemical cocktail we look for to root cuttings is the same for creating roots of seedlings or even plants in veg to create roots.

@stra8outaWeed, I think we have had similar experiences with cold grow areas. You have hit the nail on the head...it is the quality of light used not the quantity. In fact I recently had a case (in a partnered grow) were there was too much light over some clonners and getting the cuttings to root became very difficult. Rather then run just one T5 actinic over the two clonners, my partner was trying to start cuttings using 4 T5s over the clonners....too much light.
OM