cjtune is right. It depends on your case and the airflow through it. Try both. Keep a close eye on the temps. Just compare idle and load. I am using twin fans blowing on my HS and at full load it won't get over 39C. It drops back to 26 within seconds after the load is removed. This worked best for my setup.
As long as you have good ventilation, you want to blow down on the heatsink. If the fan is sucking, it's gonna grab air from where it takes the least amount of effort, and unfortunately this tactic will draw nearly NO air accross the center of the heatsink! Hopefully you can see why this is bad, seeing as that's where most of the heat will be. Now, This isn't the case for some heatsinks(alpha's come to mind). If the fan is blowing from the side, and has a shroud, then sucking or blowing, it shouldn't matter. But as long as you're blowing down directly onto the core, blowing is your best bet.