With only 1 fan - it is always better to keep it as push, rather than pull.
I have a Hyper 212 evo with only the stock fan, my 912+ has a 20cm front intake and 120 back exhaust. I have positioned the 212 evo to push the air at the back of the case, so the exhaust can take it out. At full 100% load, I get 50 to 52 C across all my cores on the I7 with room temperature around 20/21 after 1hour run. The 212 evo never spins more than the lowest setting.
But if your case configuration doesn't have an exhaust, sometimes it is better to put the CPU cooler fan as a pull, because it will be closer to the exhaust holes and vent heat out. Also depending on the case, a vertical positioning can help make use of the laws of physics - hot air goes up - and more easily vent the heat outside of the case.
To give a proper answer to your question - tell us what case do you use, how many case fans, what type of fans, etc. What is the other hardware in the computer? What type of video card? Does the video card have axial or blower cooler? One puts the heat of the video card out, while the other dumps most of it in the case.