This year we had booked a cruise in March for a cruise in November. We had placed a deposit while aboard our last cruise, also on Norwegian, and used this to hold our room. We had planned on paying the cruise off before the September 15th deadline for the final payment but things happened and we couldn't afford to do so. We cancelled the cruise and were given our deposit back.
I mention this because we have always found that booking early is the cheapest way to go. We also like to have a better selection of rooms available to choose from. We all know that things do happen from time to time so by just putting the deposit down and waiting till the last minute to pay off the cruise you can book early and if something happens along the way you can cancel if you need to.
It also makes affording a cruise easier because you can book a cruise and make monthly payments if you wish as long as the total is paid before the 90 day cut off for the final payment.