Red71 -
I believe what Scratch42 offered at the beginning of the thread is a good suggestion. IMHO, I would not wait to book.........especially if it is a popular week.
If you book early and receive early-booking savings, you will have secured the early-booking price and the week you would like. Once on board, you can check with the
DCL agent and see if the same week you want is available. If it is sold out, you already have a reservation at an early booking rate in the category you would like.
But, if it is still available and if the "rebooking onboard" savings plus any shipboard credits would be lower, you could secure the lower priced reservation and, then, cancel the first. If you wait to book on board, depending how far away your cruise is, the highly popular week you want may no longer be available.
From our recent experience, the 2005 Cat 3 Christmas Cruise that we booked onboard in 12/03 will cost us less than the 2003 Cat 3 Christmas Cruise that we booked in August of '01.

So, for us, booking onboard was a significant savings and we were still able to secure one of the most popular weeks. Plus, we always use
Dreams Unlimited, so we received their shipboard credit for the '05 cruise, as well.
Good Luck