Umm.. It's not like windows where you have to forget every once in a while and reinstall everything... There are other options. So, why not do an Archive and Install? This way, all your personal documents, emails and preferences stay where they are! You'll get a near factory fresh install of the OS on the drive and safe **** loads of time. It should work for rolling back to Leopard from Snow Leopard too.
Essentially, the installer script renames the current System Folder to Old.System Folder and a new one will be created.