I used a win 8 upgrade disk to do a clean install from win 7 on 2 of my rigs, but went into the advanced options and told it to save nothing, it pulled the W7 key on the first install, but oddly the second install failed until I poped in the W7 disk. Strange but I got both clean installs done without to much hassle.
I TOTALLY AGREE A CLEAN INSTALL IS BEST!
As you mentioned in cleans up all your junk and registry clutter. Butyou mentioned backing up your files, well a CLEAN install forces you to backup, if you don't you will have a very clean install LOL