Cruise lines price their cruises based on availability, so sailings that are selling better than planned see price INCREASES as the sail date approaches; and sailings that are selling worse than expected see price REDUCTIONS as the sail date approaches. Therefore it's impossible to determine if you should wait for a better price to book your cruise without knowing how a particular sailing is selling.
Cruise lines also offer periodic promotions which could sweeten the purchase of your cruise. Some cruisers wait for a promotion that interests them. Just be careful the cruise or stateroom type you want doesn't sell out while you're waiting for the best promotion.