To properly implement rear fill an not have it detract from the overall quality of the system, there are several areas that have t obe considered and addressed.
1) You have to account for the phase relation in the higher freqs. Failure to do so will create a bunch of peaks and valleys in the freq response. Time alignment can account for some of this. Another way to take care of this on the higher freqs where is has the largest effects is to limit the pass band of the rear speakers to midrange only.
2) You have to decide if you want fill or just rear speakers. If you want fill, you will need an amp to be able to control the volume (you want them really quiet) and the passband (200-6khZ). If you want rear speakers to "surround you in sound" be prepared to accept the consequences in the realms of imaging, staging and frequency response.
3) Music is recorded and mixed in stereo 99% of the time. Simply adding a second pair of speakers does not make it surround sound. Adding rear speakers, even when delayed and attenuated, doesn't create "ambiance" or help the feel of the music. All the "room cues" (echos, etc...) are contained on the original recording and can be fully reproduced by the front speakers. In fact that is how they were intended to be reproduced: 2 speakers in front of the listener. Unless you can separate the ambiance sounds from the recording (you can't), adding rear speakers with a delay will not accurately recreate the room but merely make it sound like you're listening to the recording in a larger room than your car. If that's what you want, go for it, but it isn't accurate reproduction.
IMO, unless you have implemented a full 5.1 system in the car and have a source unit capable of decoding 5.1 material and delivering it to the right place, rear speakers and center channels have no place in a system. That's just my opinion, your system, do what you want.