As long as you aren't switching their schedule back-n-forth, should be no problems, if all else is fine with the plant, and they're from stable genetics.

I grow seperate veg and flower rooms, and I used to run 24/0, but after a couple of months, the electricity cost jumped enough to get me to change my schedule. (fixed income...every little bit helps) Going from 24/0 to 12/12 without acclimation had no ill effects, nor does going from 18/6 to 12/12. Taking clones from a mother in the 18/6 room, you'll be fine putting the fresh, unrooted clones under weaker lights, with a 24/0 schedule.

I go with the 18/6 veg now, mainly because of the costs of running 400w HID's 24 hrs a day. But, I didn't see enough of a difference after the switch, to miss the schedule. During veg, keep in mind that you'll need to monitor moisture in smaller pots, especially if on the 24/0 schedule. Those suckers dry out pretty quick. Heat build-up can also be an issue when running lights 24 hrs a day, too.