Regarding when to use version-specific OS tags, I can see advantages for both ways (if an OS version-specific feature is required vs. if the author simply uses this OS version), but for the sake of this discussion, I’m trying to defend the latter one (although I don’t feel strongly about this; from my point of view, we could even get rid of the version-specific OS tags).
In my opinion, the author shouldn’t have to decide if a particular category of software would typically support multiple Windows version. (And while I’m not a Windows user, I’m sure there are always exceptions, for example, a text editor that doesn’t run on all versions.)
So if running Windows 7 and looking for a specific software for this OS, the question should be tagged with windows-7, even though all to be recommended solutions might probably also work for other Windows versions.
This would be in line with a simple tagging guideline like: Use the most specific tag available. (i.e., audio-editing instead of audio; and windows-7 instead of windows)
If you, as an author, know/guess that software of a particular category typically runs on all Windows versions, and/or you want to broaden the question, and/or you really need it for various versions, you are of course free to just state that you look for something "… for Windows" and tag with windows.
"I need a text editor for Windows 7"
I need a text editor for Windows 7 or later"
"I need a text editor for Windows 7 or Windows 8"
"I need a text editor for Windows 7 and Windows 8"
"I need a text editor for Windows"