Reinstall files after upgrading OS? Upgrading from 10.5.8 to 10.6.3

As I understand I need to back up my files. Is this a precautionary measure or do I necessarily have to reinstall all my files after upgrading the os?
This is just a precautionary measure, for the case if something goes wrong. If the update runs w/o any troubles (which should be the case) you don't need to reinstall all your files.

I've just configured time machine before updating my mac, and in that way you are on the safe side.