I've never felt the need to register Windows 10. It's fully usable without doing so. Might be some aesthetic customization that can't be done without registering the installation, but I've never missed any of it (and most can be accomplished via the registry anyway). I have several Windows 10 VMs I installed from scratch (downloaded officially from Microsoft) that needed no key and no registration. There's never been any nagging about registering either. Those VMs are hosted via Virtualbox on Linux, of course.
