Here's the info from the NCL web page, hope it helps:
Star to Alaska! 6-19-05 Can't wait for the Paperwork Dance!
A fixed service charge of $10 per person, per day will be added to your onboard account. For children ages 3-12, a $5 per person per day charge will be added to your onboard account; there is no charge for children under the age of three. Our crew is encouraged to work together as a service team and is compensated by a combination of salary and incentive programs that the service charge supports. It is our earnest wish that you enjoy your Freestyle Cruise experience and that our entire crew in all areas of the ship will provide you with the standard of service for which NCL is known. Therefore, if you have any concerns about the service you receive during your cruise, please bring them to the immediate attention of our reception desk staff on board so that we can address any issues in a timely manner before the cruise is over.
Both NCL and NCL America have a structured guest satisfaction program on board designed to handle any concerns raised by our guests relating to the service or on board product quickly and efficiently. In almost all cases we are able to come up with a satisfactory solution to any issues which are raised and make sure our guests can focus on enjoying their cruise vacation. In the unlikely event we cannot satisfactorily resolve the issue through our guest satisfaction program, guests will be able to adjust the service charge according to the level of inconvenience they feel they have experienced. Our clear priority is to have the opportunity of resolving the issue, when it happens, to everyone’s complete satisfaction.
Unlike most other ships in the cruise industry, there is no required or recommended tipping on NCL America or Norwegian Cruise Line ships. Guests should not feel obliged to offer a gratuity for service that is generally rendered to all guests.
However, all of our staff are encouraged to "go the extra mile", and so they are permitted to accept cash gratuities entirely at the discretion of our guests who wish to acknowledge particular staff members for exceptional or outstanding service. In other words, there is genuinely no need to tip but you should feel free to do so if you have a desire to acknowledge particular individuals.
Also, certain staff positions provide service on an individual basis to only some guests. We encourage those guests to acknowledge good service from these staff members with appropriate gratuities. For example, for guests purchasing bar drinks the recommended gratuity is 15 percent. Similarly, for guests using concierge and butler services, we recommend they consider offering a gratuity commensurate with services rendered.