Unless you receive bad service, accept it - in fact you already have, it's in the T&Cs. NCL is different because of their Freestyle approach, making it very difficult to tip everyone who gives you good service. Other lines, I agree, I always get the 'suggested tips' removed and do it myself, just out of principle. With NCL, I've signed a contract and I'll stick to it.
Thank you very much for your suggestion.
Another thought. My understanding is that the service charge is shared by the cabin stewards and waiters. Butlers, concierges and Kid's Crew are not included in the pool, so if you are planning to use any of them you might want to give them a little extra if you receive good service. Obviously bar staff get a share of the 15% added to you drinks bill.
Just noticed where you are - I can see the Island from my house on the mainland!
I have also paid service charges in advance, do they know this so do not add it to your onboard account?
They should know that, but I would take a copy of your reservation with you so that you've got proof to show the staff onboard if it should happen that something goes wrong.