I usually do a fresh install when I back up my entire home folder, which is every 6 months (Jan/July).
It's a great way to clean up all the crap I tend to download. I like to try lots of stuff on VersionTracker, etc.
I erase my entire hard drive, then reinstall off the installation DVDs only what need (I leave certain drivers off, all languages except English, etc.). Then I open up my home folder (which is now on a Lacie drive), and copy things I need back over (preferences, etc.). It takes me about 2 hours to put eveything back the way it was. It's a fun 2 hours though.
I like doing it this way because then I know everything on my computer I intentionally put there. I also don't have to mess with SpringCleaning or other crappy software like that.
I cycle several Lacie drives, by the way. So at any one time I have several backups of my home folder.