As a rule, the OS takes care of itself. An upgrade both ensures the core OS is intact, but also disables code that is too old to run.
If you would have done a clean install anyhow - lots of slowness, crashiness, etc. Then that's the reason to do a clean install. In the past (10.4 and earlier) the install process was different than the migration assistant but as the migration assistant has matured, it now runs anytime you upgrade so unless you hand migrate files and preferences, there is no difference between a clean install and an upgrade in place (assuming you select apps and settings in the assistant).
I personally love a clean install and using all the defaults for a week. Consciously installing just what I need feels "right" for me and if I end up missing everything, I can always wipe and migrate the entire setup a week I to using the new OS.
In summary, for most users, the changes in the OS have disabled the worst offenders in terms of performance, and gatekeeper keeps most old apps from running until you explicitly allow them to run. If you have performance issues, you can always flip that switch and reboot to isolate the cause of slowness.