Thx acedoc: I think that pretty much clears it up.
"Guests should not feel obliged to offer a gratuity for service that is generally rendered to all guests." Interestingly this is what we have always done on NCL. On our last cruise, our steward did a couple of extra (and much appreciated) things for me - so we did give him a little "extra". Personally I intend to take NCL at their word and only tip for what I consider "extra" (i.e. beyond what one can normally expect). I would only ever dispute the service charge in the case of a major service failure which happily has never happened to me.