This is a tough call. We booked our Dec. '15 cruise back in July of '14. They had the BOGO half off deal plus on board credit and only had to put down $100 deposit. RCCL did away with that deal and now the deal has come back, at least the BOGO half off. Not sure of the on board credit and deposit. I do know, that the later you book the greater chance you get of having a not so great room unless you are doing a suite or something of that nature or room location doesn't matter to you. I know we had pretty much any room we wanted but now they are starting to fill up.
On a side note, we have some friends who book last minute cruises and many times they get a decent deal but nothing spectacular. Depending on the cruise, they have saved from $100 to $300. Sometimes, though, they couldn't do a cruise because there were no rooms left so they just booked the next one in line. They have that luxary as we do not. The "best" deals have come when they were on the ship. Twice, they have added an extra week to their cruise for $200 per person and never left the ship. Again, nice luxary to be able to vacation whenever on a whime.
Adding to what sam_gordan said, if you book a cruise and find out it is cheaper later on, RCCL will credit your account and give you the discounted price.