In general, you would add more if you felt like it because you received great service, you would adjust your daily tip down if you were getting lousy service.

My room steward makes my bed, and turns down the bed. I don't think I've ever asked anything else from them, so I usually don't tip them above the recommended daily amount.

If I'm on a ship where we have the same dining room staff every night, and they do an excellent job, I'll give them a bit more at the end of the trip. One set of waiters once told us they know the very first night who will tip well and who won't and they focus on the people more likely to tip.

On our last cruise there were a couple of bar staff who treated us wonderfully, so we added tips to the bills there. And yes, I'm fully aware they add a tip to the bar bill already.

If I've gotten excellent service in the specialty restaurants (the ones that cost extra on some ships) I'll also add tips when signing that bill, if they don't I won't.