Clean slate installs
I always install new versions of OS X as clean-slate installs—I have it wipe the partition and do a completely new install. Then I move back my files and prefs, rebuild whatever Unix apps need rebuilding, etc.
Why do it that way? The idea is to remove doubt.
If you don’t do a clean install, and something goes wrong later, there’s that little bit of doubt that maybe the problem wouldn’t have happened had you done a clean install. And now you wonder if maybe the problem won’t be solved without doing a clean install.
You upgrade your own Mac, right? That makes you a Unix system administrator. Smart admins do as much as possible to avoid potential causes of doubt in the future. Doubt is a parasite that lives in computers.