For proper imaging rear speakers are not recommended. Music is recorded in left to right stereo with emphasis on frontal staging and imaging. Sometimes, very rarely, music is recorded in 5.1 surround for DVD Audio. Most of us don't have DVD Audio capabilities in our car (and if you do, you SHOULD have rear speakers). So for this stereo recorded music you should have a stereo speaker system.
The goal in a well designed system for best sound quality is to have the proper Imaging and good stage depth.
Imaging: Imaging has to do with left to right and center sound. The "Image" of where the music is coming from can vary depending on where the artist wants. One really good song for an example of left to right imaging is Pink Floyd's "Money" Where you can clearly hear the "chaching" and the cash register's sound move from left to right. Then you also have a center image. Most vocals are supposed to be centered coming from right in the middle of the dash, under the rear view mirror.
If you've ever listened to Nora Jones, the recording has a very prominent center image.
Rear speakers can throw off this imaging BIG TIME. Try fading your speakers all the way to the rear and have the left to right balance at 0. You will notice that you can clearly hear the left rear speaker more than the right rear speaker. Well when you have your front speakers faded in, that left rear speaker is pulling your imaging to the left. Not good