I replaced the FRONT bushings, to firm up the ride and get rid of the slop on a roadcourse. THAT was worth it.
As for rear bushings, if your old bushings have a lot of miles, then yes. For a roadcourse car, you would NOT want to do ALL of them, as this induces bind, and thats bad. The different travel arcs of the suspension bind, and make the car "feel" better, but at the limit, it binds, and off the track you go...
For drag racing, sure. It will help control the rear end and keep it where its supposed to be.
Hows that for a wishy washy answer...
