The general advice is to push air into radiators/heatsinks, as it maximizes the efficiency of the fins. Sucking air over them can cause air to flow over only some of the fins as it is drawn in from the sides.
If your talking about case airflow, then then it is usually best to draw air in from the front and bottom of the case, and expel it out the top and rear, as most cases are designed like this.
It is important to have any heatsink or radiator fans blowing air in the same direction as the case airflow so that you don't create hot-spots and bad airflow.
It is better you create a good channel of air flow. If the system sucks up air then it will flow out of the system from somewhere. So if there is a steady flow then all the warm air will be replaced by cool air instantly. So concentrate more on creating a path for the air to flow and place fans accordingly.