Unless you have people sitting back there regularly, I'd skip the install. All you need is the fronts for proper stereo imaging. Fade from back to front, and enjoy the extra juice the head unit sends to the front.
Hey Jazzcat - how are you finding your infinity's sound? Did you go with the +3 or +0 in the end?
Maybe it's where I am positioned but the rears do contribute to my overall sound experience in the car.