One caveat of booking at the last minute is lack of stateroom choice...if you are partial to a certain stateroom or area of the ship, your chances of getting that diminish greatly depending on how close to sailing date and time of year....
We typically book our 7 day cruises 6 months in advance, and never had a problem getting a stateroom we wanted. Of course, if you want one of the coveted "secret porthole staterooms" on deck 5, you must book way early...Please note that your TA will deduct any new discounts to your reservation, as they become available(if they are reliable and proactive)...