I'm not sure, but I think the cheapest times to go are either September or January. October also is on the cheap side. In Sept. there is a potential for hurricanes and in Jan. it could be cold at CC - at least the water. We went at the end of Sept. in 01 and had a great time (no hurricanes). This year it's the end of October for us.
Also, the earlier you book the cheaper the rate. Shop around at all the differnent TA and find the best deal. Be ready to jump when the 07 rates come out or find a TA that will put you on a waiting list if you decide you need a particular date/cabin.
As for crowds on the cruise, most sailings are close to full or full. Again, in Sept/Jan/Oct you might be on a cruise with a few empty rooms.