You want airflow to go one way in a system. Fans in the front pulling air in, while all fans in the back pushing air out (with your PSU in the back pushing down and out and any fans on top in back pushing up and out). If you have fans blowing in different directions...the physics is too much to explain..but its bad. Just have a unidirectional airflow.
I wasn't going to bother unless one of you guys said it was a good idea. All my fans are blowing up and out, which I already though was the best way to go.