A few notes from a first-time cruiser who booked directly with NCL... I identified the cabin category I was interested in and watched it over the course of a month or so. I saw sales during "wave season", 35% off, person #3 free, etc., and while the itemized numbers were all different, they totaled exactly the same each time. I booked about 5 months' out. Since I do not have a go-to TA, I called a cruise specialist at the local branch of the national auto club and provided my parameters; same pricing, although with a $50 OBC and a free bottle of wine. My parents, on the other hand, cruise with 8-10 other couples and get "all the deals" according to them -- drink packages, gratuities paid, OBC, etc. I don't know that they've ever seen a complete itemization as the "travel planner" usually just tells them the lump sum and payment dates.
TLDR: I did my research (as much as possible) and may not have gotten the "best" deal, but I am comfortable with the options I chose, the price I paid, and the service I received from the NCL rep when I called to book.