I always tell whoever I'm booking (cruise line or a TA) with exactly what we want; ship, date, itinerary, cabin number, etc. IMO, anything above that, best price, discounts, OBC, etc. is part of their job. At least a good agent will.
We took our second cruise, first Celebrity Cruise in 1994 on the Zenith. Even had to pay to join the Captains Club. At that time it was called the Founders Club.