This is something I was skeptical of once in my life, but for a true SQ setup, limiting the rears is essential. I still have some rears in my SQ setup, just for what is referred to as a little "rear-fill". That's my personal preference, though.
If you have rear speakers in there already, just leave them. Fade your headunit to the front, as long as you have 3 rcas for front, rear, and sub. It will take a little getting used to, but the music is clearer.
As far as your rear passengers, it will sound a lot better. Do a little test. Leave your speakers balanced and sit in the back seat, then have a friend fade it to the front. It sounds much better with only the front. Having a speaker a foot away from your head doesn't create very good imaging.
Also, the placement of your front speakers will have a large impact on the imaging. I don't know what equipment you are using, but if you are installed coaxials, then you will not receive the benefits of no rears.