Well it all depends. Think of it like this ... If you have something hot around the fan, suck out. You'll remove the heat that it's generating. If you blow in at that location, it'll just circulate the heat around the case, literally a heater. But if there's nothing hot around the fan, blow in. It'll circulate cooler air around the case and cool it off.
I've tried both ways, and since my fan was by my CPU, I experienced _way_ better results sucking out. A lot of people think that blowing in will add cooler air, which is true, but there are times where sucking out can prove to me much more effective.