I have read that you are able to clean install Windows 10 after an initial upgrade from 7,8,8.1 as your hardware gets registered. But what if you build yourself a new PC? Do you have to install Windows 7,8,8.1 first in order to get Windows 10, or is there another way?
After you build the new machine, install your current version of Windows OS (7, 8.1) and then choose the upgrade for free option. Follow the prompts and then choose not to save configuration of previous versions.
This should allow for you to upgrade but to have a clean instance of Windows 10.