I have a P182 case and there is about the same amount of clearance at the top and bottom of the power supply, so I don't think it would make all that much difference which way you mounted it. I have a non-modular power supply and I have stuffed the excess cables beneath the PSU, so I mounted it with the fan on the top.
I also followed the recommendation at the Silent PC Review web site and duct-taped the grill openings surrounding the PSU cutout at the back of the case and removed the fan from the lower chamber. This way the PSU fan draws air from the front of the case through the lower chamber and keeps the hard drives cool. My PSU (an Enermax Pro 82+) has a variable speed fan and is extremely quiet, yet still draws more than enough air to keep three HDDs at 40C or less.