I have read online (but not verified myself) than using a Windows 7 or 8/8.1 key works. This might be true or it might be misinformation. The upgrade only took 10 minutes so I did it that way with the plan to always wipe and clean install after. I decided to just jump through the MS hoops of doing it 'their way' and it all seemed to work just fine.
According to Gabe Aul's tweet [1] it looks like they do associate the device id/hardware hash so there is no need for a product key when doing a clean install once upgraded.
According to Gabe Aul's tweet [1] it looks like they do associate the device id/hardware hash so there is no need for a product key when doing a clean install once upgraded.
[1] https://twitter.com/GabeAul/status/605900073277325312
Edit: I just tried using an 8.1 Pro key on a clean install and it did not accept it.