Question: Do people not tip at all??
Answer: yes, there are many who do not tip anything at all, most people on forums like this will say they do and some of them tip extra up front (usually $10 or $20) because they think they will get better service, though there is no guarantee you will get better service for it!
Some will tell you they tip extra at the end because they had such wonderful service, if you tip the recommended amount and do not wish to add extra that is up to you... I advocate waiting til you're on the cruise and decide then.
Those that don't tip don't usually admit to it but they are out there, more than most people realise.