I did just the front two and it seem to have worked out well. I have allways had enough stopping power from the back brake and no noise. You will really like the ones on the front. If i can suggest the lyndall pads and SS braided line also. Then you have completed the package.
If you are doing the work yourself, I do the front brakes only, unless you feel comfortable with tearing the rear end down. The front are a lot easier, and they are usually the ones that make the noise, and they are the ones with the most stopping power. Some do the front for the looks, you can't see the back. I'd hold off on the back until you need to be in there for something else, like replacing the tire, that would be the first thing that comes to mind, it will be the first wear item in the rear that will need to be replaced.
I replaced both front rotors on my '05 ultra at 20,000 miles with floating rotors to stop the squeal. At 40,000 miles I replaced the rear with a floating rotor because the stock rotor had become warped. Wish I had done them all when it was new.