Update from osx 10.4 to 10.5 question?

I am going to update my g5 pcc imac from 10.4 to 10.5.

I've backed-up our file to an external. Now :

Has any one install 10.5 on top of 10.4 without formatting the hard drive.

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Is is it exponentially better to do a clean install on a newly formatted partition?

Here is the list of my purchased software:

bbedit
ms office
quicktime pro