Things to consider with rear speakers:
Time alignment-unless you use some, the overall frequency response will be worse than without them. Your midbass response will be even worse than it is now.
Soundstage-because of the frequency response issues and the fact that there is non delayed sound coming from behind you, the front stage that you want will be smeared and not well defined.
Subbass interaction-Unless you seal the back of the speakers off from the trunk, your subs will beat the crap out of them. The subs will move enough air that the woofers in the rearset will get overpowered and if you don't damage them they will at least distort badly.
I removed the stock rear speakers in my car and left the empty holes for the sub to vent through. Coupled that with a really good front stage on adequate power, and I don't miss the rear speakers at all.