It actually depends--For optimum airflow, you DO want intake fans as well. When you blow air out of the case with no intake fan, it creates a vacuum, so air is drawn in through the power supply to fill that void. Air drawn through the power supply is hot.

Most of the techs I spoke to during my A+ certification training recommended fans at the back of the case blowing air out, and a fan at the front of the case drawing air in. With an intake fan, the air that's being drawn in to replace the stuff you're blowing out is cool external air rather than hot power supply air.