New
#11
A clean install is sometimes necessary depending on the outcome of the upgrade. (My upgrade went great but I had only just reinstalled W7 after a mainboard replacement about a month prior to upgrading to W10, where as my partners computer he had Vista on it for years and years and he used the latest tech preview and it failed the upgrade and he had to clean install the tech preview, which eventually updated to the RTM, and he had made extensive backups at my constant nagging).
For me, it hasn't yet been necessary, I only had issues with permissions on the user folders, other than that, my machine boots a few seconds faster and everything pretty much works that I've tried so far. But there are people that run into all kinds of problems and will need to do that clean install after doing the upgrade
(remember that you need to do the upgrade so that windows 10 can activate itself, then you won't need to physically activate it again for subsequent clean installs so you must at least do the upgrade one time so that W10 can activate itself for the hardware in your computer).
If my computer had slowed down, or had issues then I would have done a clean install but as it is, it wasn't necessary. Although I'm sure in 6 months from now I'll want to do a clean install just to clean out the OS as I'm a messy computer operator.