If you do insist upon rear speakers, then a pair of co-axials in the standard locations, running from the headunit would be more than adequate - this gives you the option to fade them out if necessary. From an SQ perspective, any other source other than the front will alter the sound stage. The only reason the sub goes in the back is convenience - it is far more difficult to pinpoint the source of low frequencies, especially in a moving car.
You wouldn't have rear speakers on your home hi-fi for listening to CDs (surround is something totally different), so if aiming for good reproduction in-car, good fronts(well fitted) are all you need. Spending the rear speaker money on sound deadening would give better results.