I personally would book early especially if you know what ship you want and what cabin location, etc. If you wait, chances are you might be disappointed. Also, if you book through a good TA, he/she will check to be sure the prices haven't gone down since you booked and if they did, they try to get the reduced price for you. On the other hand if you are extremely flexible and don't need much notice then maybe you could wait to see what might come along.
Thursday Chat Host
I have to also agree with Mary Lou on booking early or as soon as you know what itinary and ship you want. Many holidays and school breaks do fill up fast and especially if you are looking for a quad, they are the first to go. Are you looking at any specific ships or time?