In a good system you should be able to close your eyes and picture yourself in the 5th row, front and center, of a symphony. You should be able to "see" where every instrument was during the recording. Each instrument should have a precise location, ie: if the piano is in the rear left of the stage then it should stay there throughout it's whole range. A common problem would be to hear the piano float around depending on the octive.
If you picture yourself in a symphony all the action takes place in front of you, not behind you, and you are not sitting on the stage. If you put in rear speakers not only will you lose your front soundstage put intruments will begin to float around easier and you will feel like you have headphones on or are standing in the middle of the orchestra, not watching from the seats.
You can probably put midbass speakers in the rear of the car because you can't localize anything under about 400hz.
Now all that being said, you should go for what you want. If you like the sound next to you instead of in front then throw in the rear speakers.
PS: For what it's worth, I was a sound quality judge for IASCA for many years.