Let me repeat over and over again the same answer every time this question comes up.
How much do you want this vacation? Is it a family vacation or celebrating a special occasion? How important is this vacation to your family as a whole? Will other family households or friends be traveling with you? How important is it that you take your vacation around a specific date or time frame? Is the type of cabin (balcony,outside,inside) and location on the ship important to you? Do you have special needs (handicap cabin, adjoining, connecting)? Even other special needs (physical, dietary) may need to be submitted to the cruise line weeks in advance. Is the sailing you're looking at a one time sailing, seasonal, or is it a year long itinerary? Do you have to fly to your port of embarkation? Will you have the money to pay it all at once if booked last minute? Or is paying a deposit, saving for final payment or making payments better for your finances?
These are all questions that can be arguments why you should book early. You lock in your date, ship, cabin, and location. You then can lock in better airfare by booking early. Then you can sit back and feel good knowing your vacation is planned.
You take a gamble either way. Book early - Price may go up or price may go down. If it goes down you may or may not get a refund/credit/upgrade. But if the above issues are important to you then it was worth the gamble. If you don't give a rat whether you even take the cruise, don't care about the type of cabin, have all the flexibility in the world, and can drive to the location, then you can take the gamble that the price may or may not go down. NO, you can't walk up to the pier and say "Here I am."
If you book a cruise and discover the price has gone down check and see if you can get some type of adjustment. But understand different cruise lines have different rules, and before final payment or after final can make a difference. After final payment penalties start and re-booking under a new promotion could be considered a cancel/rebook. If you bought an automobile and six months later saw it on sale do you expect the dealership to send you a rebate check? On the same token if your cruise price goes up, are you calling the cruise line to pay more? NO, your smiling gleefully that you got the better deal.