Not at all! Check as often as you like. Some people change and cancel reservations-you never know when something you want opens up.
The system does do something late at night around 1 am EST. It must update the days ADR's as it actually takes away the last day off everyones calendar. So if you had til Sept 12th earlier in the day, it disappears at around 1 am EST. Then at 6am EST the 12 and 13th of September are available for booking.