For such a major upgrade, I'd recommend backup of /home, 'just in case'. You may also want to backup your grub or lilo config (perhaps a printed copy in case you need to manually edit the file after upgrade). It saves a lot or literature research if you don't remember it all. Beyond that, your upgrade will install new kernels, libs, etc. Nothing much there to backup unless you want to restore to a known working system, in which case a full backup is mandatory. As an aside, perhaps a backup of /etc?
After the upgrade, restore /etc and have the majority of the configs you need for the applications you use. The additional configuration you will need to do to take advantage of new features will be minimized (you have the bulk of previous configs; just open the apps and look for new features).
That's my 2 cents, for whatever it's worth down under (an American saying which means "that's my opinion".