The first conclusion we can make corroborates what we found in our article, “How the Case Rear Fan Improves CPU Cooling.” Keeping the left panel of the case open can reduce
the CPU temperature in the same measure as having a well-ventilated, closed case.
Answering the question in the title of the article, we can say that the best places to install a case fan are the rear and the top panel, at least for the CPU temperature. (A fan on the left panel is said to lower the temperature of the video card, but we will still need to make some tests to check whether this claim is true or not.) It is curious that in both places the fan acts as exhaust, which means that, in our system, it was better to work with negative air pressure rather than positive air pressure, i.e. more forced air exiting the case than entering it.
Finally, remember that the results we achieve apply only for the kind of system we used, with a well-ventilated mid-tower case with the power supply installed at the bottom of the case. Different systems (with horizontal coolers, for example) may behave differently.