Dave, I think they do have ways of finding out also. I took my Stewards tip off the second day because I always tip them cash. He acted a little indifferent the next day. So I took Him aside and told Him. He was very happy. I don't know if it was because he knew he was getting a tip or if was because would be immediate cash.
When I started cruising it was cash, so I got use to doing it that way. I do leave the General Service Tip on my account because so many people help you at Lido dining area, you would never be able to track them down.
It would be a big leap for me to pre-pay, but as I am finding out on another "thread" I have out-dated thinking on some things