Merc and Bob are both correct in their own respects.
Seems to me like you're trying to get midbass into your setup by adding rear speakers. Though, this sounds like a good idea (haha, nice Pun), it's not really the best. When you do this you're just pulling the bass further towards the back, and when you talk SQ, the focus is to get the majority of the "stage" in front of you. Regardless of a "rear fill vs. no rear fill" argument (not going there), from personal experience I can tell you that you'll be better served taking your $ and putting it into a nice front stage. This includes deadening the doors, and making sure the install is good.
Otherwise, you'll drop your money on a set of coax's that aren't really made to be played free-air, nor provide the kind of depth you want from a solid midbass/midrange driver. In addition, even if you put those speakers in the rear deck you're still going to have to add some deadening to tame resonance and flex. Otherwise you've basically sat $100 on top of your rear deck and called it a day.
This is all just my opinion...do as you like, really. I'm just speaking on personal experience and going by what I like in music. I'm just thinking that your rears aren't what you're really wanting here.