So, I was wrong, I guess "reserved" doesn't really mean reserved forever. From the Windows 10 FAQ on Microsoft.com
"To take advantage of this free offer, you must upgrade to Windows 10 within one year of availability. Once you upgrade, you have Windows 10 for free on that device."
You can also revert back to Windows 7 if you don't like 10. So, in order to guarantee that you can install Win 10 a year from now, you should install it at least once on the PC you want to put it on eventually then go back to 10 - if you think you won't do it until at least a year from now.