Also, what is the farthest out that you can book a cruise? Is there any advantage to that, say any savings?
I don't know how early you can book, but the advantages to booking early are potentially lower rates, and maximizing your stateroom selection options. I am in the process now of booking our cruise for next June, and while I knew there was a rate advantage, I was a bit surprised at how much the stateroom selection was already limited. (Chalk it up to cruising during a "popular" period.)
A complicating factor in my case is that for my family of 5, I needed either a Cat 4 or connecting lower category rooms. I wasn't going to insist on a veranda, but I did want outside rooms, and with the ages of my children, I wanted connecting, not just adjacent. To my surprise, there were no connecting rooms left in Cat 8 or 9, and even the number of remaining Cat 4 rooms was less than I expected.
I would suppose that this wouldn't be as big an issue with a party of four, but if you have more stringent requirements, such as a particular deck within a particular category, or you want one of the "secret porthole" rooms, etc., then there is a major advantage to booking well in advance.
I was looking at the price difference between the end of May and the beginning of July, and it's about $1000 for 4 in a room!
I was initially looking at a June 6 (2004) departure on the Wonder, but later decided that June 13th would work much better for my family. That one week change raised the rate over $200. Slipping another week would have added a similar increase over the 6/13 rate. I would assume the rate structure conforms to some sort of curve reflecting the popularity of the cruise dates (supply and demand).
Good luck with your planning!
I got some sad news and found out that we are not going to be able to go to WDW for three days before our cruise in May 2004. Now we have to figure out if we are going to fly to Orlando the night before the cruise and get a hotel. I want to do it this way and DW wants to wait until the morning of our sailing. I have noticed with past threads that the vast majority of you have suggested that one should fly in the night before and make sail away day relaxing. I agree and would like to ask if anyone has any good suggestions on where DW, DD, DS and myself can stay for an enjoyable Friday night before our cruise? Also, any suggestions on activities to keep DD (7yo) and DS (4yo) happy the night before cruise? Thanks for your help and suggestions. 
