it is worth taking a tuxedo if you would like to - otherwise it isn't. It's not like on many other lines where there is just the possibility to avoid participating in the formal dinners by choosing the buffet restaurant.
Assuming that you will be going on "Norwegian Sea", one of her main dining rooms will offer the formal dinner once or twice per cruise, while the other one will offer a similar if not identical menu but without the need to dress up. In addition to that, for a nominal fee, one can also have dinner in the alternative restaurant, Le Bistro. I would recommend going there at least once, as from my experience (not on "Norwegian Sea" but on several other NCL ships) Le Bistro offers a very good dining experience that is well ahead of what is offered in the regular dining rooms (which I would consider standard cruise fare that is neither bad nor particularly memorable).