If NCL is like most of the other lines, if you have the automatic tips removed, the steward and waiters will have to turn in any money you give directly to the common pool. If you use the automatic tips, they can keep any extra money you give them.
I have been on 22 cruises on various lines and have always had great service. I consider the $10 a day a fantastic bargain and usually give a bit extra.
It is my understanding that the stewards and waiters receive only $50 per month from the cruise line and work primarily for tips. People who refuse to tip do nothing to change the mode of operation of the cruise lines. Rather, they only hurt the hard working (70 hours or more a week) people who serve them.
I would not hesitate to call anyone who refuses to tip a cheapskate.