If you have a particular cruise on a particular date with a particular cabin in mind, the passage of time works against you..

There are large TA's which buy blocks of cabins and then resell. If they can't sell a cabin at retail, they may sell it last minute or even make it available through a discounter. This lot may also include cabins from last minute cancellations. The latter are either pure or close to pure profit because they have been paid for.

Similarly, there may be special "deals" which may or may not be available later on. These might include better cabins, various packages and the like. If you care about these things, you should grab them. But, if you don't, then don't sweat it.

You should also consider that last-minute airfare may be very expensive (or not). It does you little good to save on a cruise and then spend more than those savings to get to your departure point.