i usually install them so that:
1 - i can read the label with the p/n and specs when i take off the side cover
2 - the cable management makes sense for me, especially modular plugs
3 - if i use a right-angle AC power cord, it's exiting in the preferred direction
4 - if i need to use the power switch, it's more accessible one way than the other
5 - if the fan is pulling through bottom of case, install thin filter sheet if case doesn't have one to begin with