The benefit to running lights 24 hours a day during veg is simple: faster, more vibrant growth over the same amount of time. The only reasons to use anything other than constant lighting during veg is if you WANT to slow down growth (because your flower box/room won't be ready for a while), or because your ventilation is inadequate to handle the heat without a cooling off period.

So, if you don't have good ventilation, or if you'd rather grow a bit more slowly for some reason, use 18/6, or 16/8. If you do have good ventilation and the heat isn't a problem running the lights constantly, and you want faster and more vibrant growth, keep the lights on all the time. You don't really spend more money, because you can get the same size plants in 24 days of 24/0 lighting as you can in 32 days of 18/6 lighting. You spend the same amount either way, you just spread the electric bill out over a longer period of time by using less light per day (but for more days).