When booking a cruise, I always suggest booking it as soon as possible. It's possible that the rate you're seeing is the cheapest it'll ever be available for you. Prices could go up at any time, plus the longer you wait, the less options you have for booking a cabin you'll like.
If you book it and the price goes up, you're protected against any increases, except for taxes or fuel supplements. If the prices go down, most of the time your agent can get the better price for you, unless the final payment due date has passed.
So the bottom line is that if you see a cruise you want, book it! It's better to get it now than to wait and hope you'll get a better price only to find the price has gone up and the cabin you wanted is no longer available.