This isn't Windows. "Better" is subjective here. There were some things along the way that you could only use if you upgraded. If I rember correctly, the version that officially dropped support for Classic was still able to run it if you had upgraded. There have also been various apps over the years that an upgrade install wouldn't remove, so you could keep using them, but you wouldn't get the with a clean install.
All that aside, Apple's upgrade process actually works. In my experience as a highly skilled bench technician, I found that every time someone brought in a computer wheret hey'd done the upgrade themselves and it failed, it always came down to a hardware problem--usually either a bad hard drive or bad RAM. The difficulty with making this claim is that there isn't a good hard drive testing program for the MAC (so in forums, people will dispute that they have a bad hard drive because all the Mac tools tell them it's fine); we always had to pull hard drives out and test them on PCs. Obviously, this annoyed the Mac purist in me, so I wrote my own by modifying e2fsprogs' badblocks tool.
The moral of the story is that no, it isn't decidedly better to do a clean install on OS X. Unless you explicitly want to get rid of everything you've already done, you're just making your life harder and wasting your time.
