This question was answered for me by Don Burn on MSDN, but I wanted to post the answer here for SO users:
Windows Server 2008 R2 == Windows 7
Windows Server 2012 == Windows 8
Windows Server 2012 R2 = Windows 8.1
As a rule you build for the earliest version of Windows you wish to support, and the driver will run on all versions after that. There are special cases where there are new capabilities that you may want to take advantage of in the newer system, but that is the general approach.