529 distributions have to be made the same year the tuition is DUE. In other words you can get a bill in Dec for Jan tuition, room & board. You will want to keep things simple and take out $ in Jan.
In my opinion I would take the funds out of the 529 whenever you are allowed to do it. Balance it with the American Opportunity credit, you want to use that the first 4 years of college if you can. There are income limits however.
Having $ leftover in the 529 after college is completed is not a benefit. Grad school might be paid by employer.
