I think I have said this before, but I will repeat it one last time...
The $10 a day service fee is NOT a tip. It is NOT going to the staff. NCL was very clear on that during the seminar I went to. It was originally instituted to help pay for extra staff benefits that were mandated by the American crew -- like vacation, medical plans -- but they decided to institute it across the ships to help cover "rising costs in crews." It should NOT have anything to do with how and when you tip.
I think we're going around in circles here. As a TA I feel cheated by NCL not including this in the cost of the cruise and I think they are really taking advantage by adding it to voyages, HOWEVER, I don't want to see the very hardworking crews cheated because people refuse to tip thinking the $10 a day is going to the staff as a tip.