My understanding (and it could be wrong) is that when fares are released they are what they are and tend to increase as you get closer to the sail date (as people book the rooms). Supply and demand. If the rooms aren't booking fast enough they may have a sale and/or reduce the fare to attract some more people to book.
If you want a specific room that is in high demand or a type of room that has a limited number available then you should book ASAP.
If you are traveling over a holiday or school break book ASAP.
Since you are looking at dates that are during the school year and don't seem to be around any major holidays (other than Halloween) you might be able to get away with waiting until the last minute to book.
Check when the paid in full date is for the cruise you are interested in. If you book now and the price drops before that date (and you personally have not paid in full yet) you can call
Royal Caribbean and get a price adjustment.
While cruises out of Miami may be less expensive you will need to factor in the additional travel time and cost (car rental or flight). For us that ended up making the cruises cost roughly the same so I just went ahead and stuck with Port Canaveral.