Unfortunately there aren’t “one size fits all” answers to your questions but here goes:

Some lines, particularly the mass market lines, cut off new bookings 24 to 72 hours before the sailing. Yet a few lines, like some of the luxury lines, literally take bookings right up to the sailing. I’ve heard all sorts of reasons and excuses for this from TSA/DHS wanting passenger manifests 48-72 hours ahead of US departures to lines cutting off new bookings because they are in the middle of assigning guarantee bookings to concerns about provisions. Not sure any are true or accurate.

Most lines will honor price reductions after booking but that comes with a lot of caveats. One, if the price goes down before the line’s final payment date, which varies from 60 to 120 days before the sailing, then usually you can get the reduced fare. I say usually because you’ll also see sales or added incentives that have the fine print of “for new bookings only”. After final payment all bets are off and your chances of getting the reduced fare go down significantly. Yes some have complained when this has happened and some lines will either lower your fare or throw in something an onboard credit but not always. Two, there are some lines, particularly in the luxury market, that have fare guarantee programs that basically “insure” you’ll get any reductions literally right up to sailing. Of course even those will sometimes drop the “new bookings only” fine print but not often as that flies in the face of their fare guarantee.

As for booking early. We always do because we have to plan our cruises so far out because of work. Not so bad now as I’m semiretired so we’re only planning around my wife’s job. Luckily we’ve never had a cruise where we booked early only to see our cabin category go down in price and not been able to get the new fare or some other compensation. More recently, like the past two or three years, we’ve actually seen prices go up which of course doesn’t impact us. Another general comment is pricing and price fluctuations really are driven by where and when you’re cruising. If you’re doing a Caribbean cruise where a cruise line starts in say October to November and runs the same basic 7-day cruise week in and week out until April then you’re going to see prices all over the place. Conversely you’re going to see a unique, one off itinerary for say a South Pacific or Med cruise that is not only expensive to start with but stays that way and may even get more expensive. The other aspect is for these limited or unique cruises you run the risk that if you wait too long to book you’ll see the ship fill up and not be able to book. All comes down to where and when you want to cruise and how flexible are you about a line, ship, or cabin category.

Oh and as for cruise lines saying you should book early....,sure they do. They get your deposit money in their account, they are planning and hoping that once you book you’ll feel committed and won’t cancel, and they like to see cruises fill up early so they don’t have to reduce fares or offer additional incentives. What I like to see, and it has happened to us, that the line will offer an upfront incentive to book early.