On Stack Overflow, a version tag (e.g. "c#-4.0") is used when the question is about a feature specific for that version.
Is the same true for the OS tags used on Ask Different, or should I use the OS tags only to report the OS I am using?
Our version-specific OS tags use the 'big cat' name of the OS release.
10.4 -> tiger 10.5 -> leopard 10.6 -> snow-leopard 10.7 -> lion
Use one of these when you have a question that's OS-specific.
When your question is not OS-specific, use the generic
Tags should not be used just to tell us what version of the OS you're using; add that to the question itself.
"How can I rename Desktops in Lion?"
Desktops (Spaces) changed a lot in Lion; this is a very Lion-specific question.
"Error when connecting to my Mac with SSH"
This is a general troubleshooting question. While it may be useful to know the OS, it isn't a question specifically about that OS.
[osx], version info in the question.