Unless you have some other reason for wiping your HD clean with a new install of Leopard, there should be no reason for it. I did upgrades from Jaguar and Tiger on a few machines from a family pack license, and all of them were upgrades that went smoothly.
The only annoying bit was that my printers were lost and I had to re-add them.
Clean install lets you start off from square one. Everything runs as if the computer was brand new. Back up everything because the hard drive will be totally erased during installation.
When going through the pre-installation windows for 10.5, click the options button when you get to the right window (sorry, but I don't remember the order of them) and choose "Erase and Install." Then you will get a clean version of OS X on your computer. Keep in mind that you will need to reinstall all other programs (including iLife).
There is an options button somewhere in the bottom left when you are being guided through installation setup. You can choose upgrade/archive/clean. I just did archive though.