I'm partial to the 18/6 schedule for two reasons: 1) As Image Reaper suggested, in nature, plants don't get 24 hours of sunlight. I've never had substandard results from imitating nature, and don't know anyone who has. 2) Your power bill will thank you for that extra 6 hours of downtime if you're running 1k of lights or more, several ventilation fans, humidifier/dehumidifier, etc.

One upside to keeping a 24 hour schedule is that you don't have to worry about fluctuating temperatures. With fluorescent setups, the 24/0 schedule for vegging plants, clones, and mothers seems a bit more feasible. But still not entirely necessary.