This is a hotly debated topic around these parts. Some say it is two different things. Some say since the Service Charge replaced the old "envelope tipping system" that the Service Charge is indeed the tips. Some will say it is semantics, some will say phooey. 🙂
You are under no obligation to tip above and beyond what the automatic charges are. If you choose to do so feel free. That is your business and no one elses.
Here are a couple of links from NCL.com FAQ's that may help:
Do what you feel is appropriate. Personally I rarely tip above the automatic charges.