Got my case; fan positioning?

Ok so I just got a black case that looks ok, but hey for $23 I've not much complaint. It came with two fans on the side panel , lower left, that I think blow inward. I got my friend to give me another fan with it so I could draw air in at the front. One of the techs in there was saying I should NOT put a fan on the back vent, because the power supply is already drawing air out the back. and with my power supply, it's going to have 2 fans, so I guess one will pull up from the bottom and out through the back.

Do you agree that I shouldn't put a fan on that back vent?
Also, should those side panel fans stay as intake, or should I flip them to exhaust, with just the front fan pulling air in?