We have the same issue in the SQL Server realm. Unfortunately it's not possible to make a version-specific tag mandatory, since not all questions are about a specific version, and others can be about many versions (IE 7, 8, 9 and 10 let's say - not much room for other tags then).
The other problem with tagging as a specific version is that can appear to become stale over time. A new version of IE comes out, okay, is this question (and more importantly, its answers) still relevant? Who keeps its tags up to date, or updates the answers to account for new versions?
Again, a SQL Server example: we often get questions closed as duplicates of really old questions, even though newer versions of SQL Server provide much better ways to do something. In those cases it would be nice to go back and say "this question is good for version x and lower" - unfortunately often newer answers have been added, and it's only the most highly up-voted and/or accepted answers - which have been there the longest - that are no longer the best way to do things in current versions. Still, I feel that in general, tagging with a specific version is usually only useful when it is about a feature or problem that exists only in that version. And that is pretty rare in the technologies I work with.
Tagging with a minimum version is often more important, since features added in version x usually also exist in version x + n, but I still don't think it is necessary or should be mandatory. When I know an answer may depend on version, I ask. This is almost certainly going to be more effective than trying to automate forcing them to specify a version in any way.