In Windows, things like the Registry get cluttered and carried over from OS to OS. On Macs there is no such thing, but instead you have folders in User/Library/Application Data that contain all kinds of remains of old apps. You may want to manually delete all the ones that were created by apps you no longer have instead of doing a clean install.
When upgrading your Mac OS, your entire system folder will get replaced by the new system, so there should be no problems with the system itself. The only think you may want to look into are the various preference files, corrupt fonts and cache files that your computer has accumulated over time, that you might want to get rid of.
If you do a clean install, make sure you don't migrate your data back from Time Machine, as this defeats the purpose of a clean install: Your Application Data will get restored, just as if you didn't do a clean install.
I think that the smartest thing to do is to simply do an upgrade, see if you're happy with it, and if not you can easily do a clean install from the recovery partition.