I personally don't run rear speakers because it throws the balance off so badly. With the way they're mounted in the cab it's next to impossible to tune the time alignment (or balance and fader controls if you don't have a deck with t/a) to get decent imaging all around. It with either sound OK to everybody, or really good for the driver and horrible to the passengers. The reason you can put a sub anywhere and make it sound decent is the way your ears perceive the lower frequencies. It's much easier to "locate" the higher frequencies with your ears.
And if there are any back-seat passengers, those speakers are less than 1 ft. away from one ear, while the others are all the way across the cab. Not my idea of a pleasant listening experience to only listen to the one blasting right into my head. (This also goes for the front seat of a RC, which is why I took out the first aftermarket set of 4x6's I put in there after 1 drive. Waste of $80.)
I run a Pioneer 960 which has time alignment. I'll never have another deck without it because of how incredibly adjustable it is. You don't need to use balance and fader controls anymore. I have mine tuned so that sitting in the drivers seat it sounds like everything is coming from directly in front of you through the windshield. Just like the band is setup on your hood.
I'd also look at infinity for 4x6's, they make a couple of "plate mount" speakers that are basically a 4" woofer with a tweeter off to the side. And they're the best sounding 4x6 I've found for under $200. Usually you can find them 50% off on some of the audio websites so under $100 for the pair.