That's really up to you.
In my case (the Enlight 7237) the rear mount is just below the power supply in-line with the cpu. This is a good place for an exhaust fan as it is pretty high up in the case.
The general rules of thumb are as follows:
1. equal amounts of intake and exhaust
note - some cases have a lot of ventilation holes
which allow you to have more exhaust if desired.
2. Exhaust fans at or near top of case (heat rises)
3. Intake fans should be at bottom of case.