No, the best setup is to have some blowing in and some blowing out...it's not going to be blowing hot air into your system because the system will be hotter than your room, so you'll technically be blowing in cooler air.
Generally you want the front fans as intake, the side as an intake, the rear and top as exhausts.
With both intake and exhaust you constantly get movement of air through your system. If all you had was say one fan, then you'd want to put it in the rear as an exhaust. Generally it's a little bit better to have MORE exhaust than intake, but not a whole lot.
Point being for the best optimization you need both intake and exhaust in good places so that there's a nice stream of current.
Also they have duct mods where you can connect the CPU fan to the hole in the rear so outside air goes directly to your heatsink.
Cools down the CPU quite a bit.