All of the "Rules of thumb" on when pricing is supposed to go down and when it's supposed to go up are guesses. You cannot predict it ant better than you can the price a stock will be tomorrow or whether it will rain two weeks from Sunday. Trends happen and people take those trend to mean things will always be the way they were when they saw the trend. As CF says, everything is based on availability. Late last year the winter took a bitter cold turn in the NE U.S. I can't tell you the number of calls asking why it was double last years rates to book a cruise from NY ports. Everyone from Toronto to DC wanted on a ship and out of the snow and bitter cold. Prices soared and even getting a cabin was dificult. I had people in Boston calling after the sailings in NY were sold out asking for sailings from FL. Problem was there were no flights available at any price.
My recommendation has always been and will always be, book early and take advantage of the early booking prices and incentives. If your cruise does what it did the year beofre and drops call your T/A or the cruiseline and get it adjusted down. If it climbs, you're protected. Be aware that with HAL, NCL and PCL they will not adjust the rate after the penalty period begins (typically around 75 days before your sail date). Carn, Celeb and RCCL all will continue to make adjustments after penalty.