Depending on the TA and when you book the cruise, the TA may have already returned their batch of cabins to to cruise line control. So it might not be any better booking with a TA unless you have an established relationship with them.

I booked a cruise through an on-line TA thinking it was a good deal. It was inside the final payment so I made full payment at booking. They gave me a ship board credit. The next day the cruise lines own site had the cruise cheaper than I paid including the on board credit. The TA said they could not do anything about the price difference but that I would be given a cabin upgrade, which I considered a worse cabin that I had, it was just on a higher level. So no real advantage using a TA in my case. And the customer service sucked.

MY PVP always gets me the best cabin for a great price. I have no hassle getting price drops or on-baord credits since I can do them myself and not wait for a TA to get around to it. If I have any problems I can deal directly with the cruise line.

I have learned to check the cruise lines own website and compare it to the Ta's quote. It may not be a good deal if you do not know the TA and the prices are close.