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.
Example questions
"I need a text editor for Windows 7"
windows-7
I need a text editor for Windows 7 or later"
windows
"I need a text editor for Windows 7 or Windows 8"
windows
"I need a text editor for Windows 7 and Windows 8"
windows-7 windows-8
"I need a text editor for Windows"
windows