SPCR

Of late the PSU mounting position has moved from the top to the bottom in many cases. A trend that probably started with the Antec P180.

My question is fairly simple, although the answer(s) might not be: How important for cooling, noise and so forth is the bottom PSU mounting position in a case? Is it always preferable to the more common top mounting position or is it just one of these things, that are nice to have, but not really vital for a good case.

Thanks in advance for your collective enlightenments.

Hot air rises.
This hot air gets into the PSU if top mounted, increasing its temperature.
An increased PSU temperature (typically) increases the PSU fan speed, increasing noise.
Just my thoughts.

In the case of the P18x it's more about having a separate air flow for the PSU (and HDs) than whether it's on top or bottom. They created a separate chamber so the PSU fan draws air from the outside (through the front and over the HDs), instead of drawing air from around the CPU and MB. This insures that the PSU gets cooler air going in to it, thus less fan speed is needed, thus the PSU is quieter.

Here's a old but apropos discussion about this topic: Does "Evacuate the heat" role for the PSU work?

Thank you all for your input and especially MikeC for providing some interesting reading material.

Also usually the hotter a PSU runs, the more wall current it wastes.