Should my computer fans be sucking or blowing?

Depending on your chassis configuration will determine whether or not the fans should be blowing or sucking. Computer cases that have only one fan in the back should always be blowing the hot air out. Another common configuration is to have the fan in the back with a fan in the front portion of the case sucking in air in.

As can be seen in the picture to the right the computer case brings in cold air from the front of the case because it's always open and not constrained and as the hot air raises in the case it is blown out the back. The important thing to remember is that if you have more than one case fan at least one should be sucking air in and one should be blowing air out.