Generally always...
But particularly in a sedan/coupe...
I advocate fully removing them from your rear deck... (that is conditional, of course..//content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif your front stage has to be up to snuff)
You absolutely will find more sonic benefit if you take the budget that you are allocating to rear speakers, and reallocate it to the front stage effort:
Might be new speakers, might be developing a baffle/enclosure for them, might be kickpanels or angled door pods, might be improved amplification, crossover strategy, etc..
Whatever the weak spot is.
There are many reasons "why"...
I wrote this site to illustrate the science behind the sound, and it details the inherent difficulties that are the reason that I don't recommend running anything in the rear:
http://www.betteraudio.com/geolemon/Phasing/Phasing.htm
You can only take your system so far, after all, with inherent known flaws in the design...
And that's not to say that a system can't be built to be great with rear speakers.. but you need to address these flaws in other ways... quite difficult! //content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif
Also, don't be fooled by thinking that "I turned my fader all the way to the front, and it sounded thin and bad" means that "running no rear fill sounds bad"...
Remember, that's why I said it was conditional... the front stage must be up to snuff.
I'd personally rather redirect funds to that effort, rather than actually spending money on something that inherently compromises sound quality.
And my favorite argument is "but what about the rear passengers?"//content.invisioncic.com/y282845/emoticons/biggrin.gif.d71a5d36fcbab170f2364c9f2e3946cb.gif