There is really no way to answer your question. Prices fluctuate according to demand whether it is an agency or Princess. The only way to know is to comparison shop; and make sure you are comparing the same catagory cabin on the same ship on the same sailing.
My personal experience has been that I get the best price by booking very early (a year or more) directly from Princess. I get mailings from several agencies that I have used and out of curiosity I compare the prices with princess.com and Princess is always cheaper.
But I am sure others here have had different experiences. If you absolutely have to have the lowest price then you are going to have to do some legwork. After many years of cruising my approach is, if I see a cabin I like at a price I like, I take it. There are too many variables to go hunting for the "best" price.