Ideally, if you only get one set, screw rear fill, and get a GOOD set of front speakers for the soundstage. many competition cars have only front speakers for soundstage since rear speakers are distracting and tend to drag the sound stage too far back.
if you have the money and have people in the back of the car much, sure, go with fronts and rears, but put goot separates in front, and coaxials in back.