Actually gratuities that are included in the fare, are not tips, gratuities on non Australian cruises are the American way of paying a persons wage.
I think the rational is if you really do your job well people will pay you more.
Here in Australia and many other places we pay a reasonable base wage.
Tips are for outstanding service here.
On the ship, as cruises from here include a working wage we do not pay the gratuities.
Tipping is very optional, depending on what you feel like giving, and what is within your financial means to give.
If you feel that someone has made your trip special, a tip is appropriate, if you can afford it.
Other wise as lyndarra has said make a special effort to make sure princess knows that they made your trip special. It also helps them having their contracts renewed.
Regards
John
and PS: Yes welcome to CC