You're absolutely right - never stop checking the rates as they can change at any time. This is especially true if you see or hear of any specials or sales that are going on.
But your agent should have caught the lower rate because if they're any good, they will always check the going rate before they actually make the final payment just to make sure the price has not changed.
But even after final payment has been made, keep checking the rate. If the price goes down, notify your agent immediately. While most cruise lines now have policies whereby they won't honor the reduced rate with a price reduction on current bookings, occasionally they'll offer onboard credit or an upgrade. Of course sometimes this may also depend on the agent and their standing with the cruise line.
45 Cruises & Counting! Favorites: Paul Gauguin to Tahiti: Uniworld River Cruises in Europe; any of the Celebrity Solstice-class ships; Holland America for 12-nights in the Baltics & Russia; RCCL for 14-night Greek Isles, Turkey, & Croatia; Holland America for 14-day Alaska cruisetour; 10-night Canada/New England cruise; 21 days in Hawaii including a 7-night NCL cruise; Oceania for 25 days in Asia; & 3 months touring Europe by train. And many days spent in all-inclusive resorts!