I've never felt pressured to give more - I'm not sure they know who is pre-paying them and who isn't (and there are enough who would get AI and then demand that the tips be removed I doubt they would put much stock in any such list.)
I've never bribed my stateroom host, servers, or any bartenders and I always have great service.
While I may - and have - give extra money on the cruise, I know that FAR more valuable to them is mentioning them by name on comment cards and/or a letter after the cruise. That can lead to promotions and raises that are worth far more in the end than a little bit extra as a one-time thing.