There is no argument generally with this subject... 18/6 veg then straight to 12/12 for flower. Keep it simple.
That certainly is how many people do it, and its pretty tough to go wrong that way, but I'd say its not quite that simple. It depends a little bit on your setup, the kind of plants you're running, and what you want to accomplish.
If you're running autoflower plants you can run them 24-0 from seed to harvest if you like, though many believe you'll see better results with SOME dark time (ie 20-4 or 18-6).
Assuming you're running conventional photoperiod plants, its perfectly fine to run your lights 24-0 in veg if you like, and the plants will grow bigger during that time than they would if you kept them at 18-6. Veg period plants don't "need" a rest time, though many people still like to give their EQUIPMENT a rest.
Whether 24-0 is better or worse than 18/6 depends mostly only how much space you have and how big you want your plants to get.
If you're trying to minimize plant height and/or also get to harvest as soon as absolutely possible, you can run the plants 12-12 from seed to harvest.
In terms of decreasing light, again, some people think you'll get better results if you gradually decrease the hours of light over flowering (like what happens naturally as the summer turns to fall, and then to winter) rather than dramatically switching to 12-12 and keeping the hours like that until harvest. Herijuana, in particular, is one strain that some claim work better this way.