Install 10.6 over 10.5 on iMac
To offer you a different point of view: I assume you're going to have a Backup of your system before performing such a drastic operation. 

If you do have a backup, I strongly advice you to install Snow Leopard from scratch and then, using migration assistant, transfer your data from your backup (Time Machine or clone).

I have upgraded seven machines from Leopard to Snow Leopard. Three with the method you describe and four with a fresh install. 

I had problems with two of the upgraded machines (that eventually had to be reinstalled) and zero problems with the fresh installs.

That's my experience and ymmv, but bear in mind that the changes to the underlying system between Leopard and Snow Leopard justify a fresh start :)

You can always try the upgrade, but always have a backup, there are a lot of things that -depending upon your system- can fail.