Good Practice When Writing a Question
The tag wiki is accurate. Version-specific tags are for issues that don't apply to all versions, or questions about a specific version. In those cases, the version-specific tag is important information about the nature of the problem. If the problem is version-specific, adding the version-agnostic tag is just confusing and defeats the purpose.
Questions about a problem concerning, or specific to, one version: use just that version tag.
How do I do XYZ in Win XP?
I just installed Win 10 and nothing works anymore.
Questions about a problem or feature that applies just to a range of versions, like something introduced in Win 8 that also applies to 8.1, and you know that the issue applies to several versions: include the applicable version tags. It could be helpful to others searching for a solution.
Questions where you don't know whether your problem is version-specific: use the version-agnostic tag and mention your version in the question. However, this is a case where nothing would be lost in including both tags.
Generic question that applies to all versions (or at least all non-ancient versions): use just the version-agnostic tag. Adding a version-specific tag just confuses things.
It's generally a good idea to mention the installed version within the question.
Retagging an Existing Question
If better tags will help the question get answered, or help future searchers find it, retag it.
If better tags won't make a difference, don't bump the question just to perfect the tags. This would include new questions where the tags are "good enough", and old questions that have already been well answered despite "sub-optimal" tags.