I booked our Island Princess cruise to Hawaii about a year in advance. From the time of booking, the prices only went one way--up. I'm not saying this will always be the case, but as the others have pointed out, if they should go down, you can get a refund or credit.
And even at a year out, a lot of the mini-suites were already booked. If you are considering Golden Princess, she has over 700 balconies and a whole deck of mini-suites, so getting one shouldn't be a problem. But if you want a specific cabin, like I do, the earlier the better.