As for rear fill the choice is yours. Personally I talk to my passengers and only listen to music when I'm on my own. I'd rather have better staging for myself, since the car is mine and I rarely play music with passengers, rather than improve the sound for those very rare times I play music with people in the back seat. Remember, stereo music (Radio, CD's, MP3's etc) comes ONLY in STEREO. ie 2 channels, left and right.. adding rear speakers is not giving you surround sound, or anything other than a messed up sound stage.
However, if you want to please your rear seat passengers then go for it... but it's at a slight detriment to your listening when you are by yourself...