rear fill? yes or no

IN GENERAL:  rear fill is for people sitting in the back seat.  If you don't plan to carry passengers very frequently rear fill simply tends to make creating a proper front seat sound stage more difficult.  Surround sound is, of course, different and requires rear speakers.

I agree with both the Spiz and DYohn. I love my front stage now that it is powered /setup properly. I don't even miss my rear fill.

The rear passengers, which isn't often, hate not having speakers back there...

You have to provide a gread amount of attention to the rear fill if you use it.  For instance, in most cases the backs of the rear cones are subject to the direct air pressure from the subs.  You have to provide a good baffle that will separate the rear speakers from the sub cavity, and the baffle has to provide a good deal of airspace so the speakers can behave as if they're infinitely baffled.  (Small enclosures or "hats" are too small for proper speaker function.)

You should also take a good look at the angle that the back parcel shelf provides.  Does it face straight up, or does it tilt more towards the front of the car?  Some sedans provide a strong tilt toward the front, and those cars are also usually factory-supplied with a back baffle.  So those cars are the best candidates for using rear fill. 

If rears are mounted in the rear panels or doors, those panels have to be damped just like at the front door speaker locations.  That is something that otherwise may not be necessary when damping the car against sub tensions.  If the damping is not done and rears are used, they will likely lack midbass punch in the critical sub-mid crossover region.  Just like the fronts.

I believe the front soundstage in my car is slightly elevated by adding a proper mix of rears.  Of course, I could always just disconnect them and find out for sure...but why mess around with something that sounds right?  (...and I already went through all the trouble that I just described!...)