We are frequent Princess cruisers and our TA always gives a 7 to 12% fare reduction and if she has a block of cabins on the cruise she throws in a modest OBC (between $60 - $80). On two other lines she didn't reduce the fare but did provide a healthy rebate following the cruise and some OBC.
If considering using a TA to book a Princess cruise, realize that they are not allowed to advertise their lower fares. Instead you must inquire (phone, online, in person) for a rate quote.