It really depends a lot on the line and the itinerary whether they change ship's time. As others have said, they publicize it well.
Also note that on just about all of the newer ships there's an LCD clock on the telephone. This clock is slaved to ship's time, so if there's a time change, the clock changes. This is useful because it's also linked to the automated wake-up calls, so if you program a wake up call it will ALWAYS be on ship's time, whether there's been an adjustment or not.
Naturally, on long voyages, and especially crossings and Canal cruises, they do adjust the time, sometimes for several days running, so that you're not all goofed up when you get where you're going.