I was wondering the same thing myself. Seems like the ideal thing to do would be to have the lights come on a few minutes later, and off a few minutes earlier each day, but I don't know if it would help the plant or not. I did read an interesting article the other day that said to bring the plants down to 10 hours of light per day. The reason given was that Cannabis is a 'short-day' plant, and while 12/12 will make it bud, it will always be on the verge of re-vegging, thus making it take longer to finish, and more vulnerable to problems from light leaks.
Don't have a clue how accurate that is, but it's what I read and it sounds like it makes sense. I'm thinking of taking mine down about 5 minutes a day until I hit 11/13 just to make sure the plants understand what I want them to do.