There are many factors, heat and humidity both come into play. Also keep in mind a pusher fan will never be as productive as a puller, and the A/C fan, even re-wired will not push the CFM that the stock main fan will.
With that said, we are lucky enough to have a large frontal area for air to push through the radiator, so location (state), and it's heat/humidity both come into play a bunch.
I always err. on the side of caution and IMO, it is always better to be safe than sorry. With the frontal room we have between the radiator (or condensor), and the FMIC, there is no reason a secondary pusher can not be fitted for safe measure, and I would highly recommend doing so, especially with a DSM radiator that is about 1/2" thinner. With these cars, we are also lucky enough to have a 90-amp alternator as opposed to a 65-75 amp unit on the DSMs, so the power draw will not hurt any either.