Currently CiviCRM.SE doesn't seem to have a policy on 5.x version tags.
As questions are now using these tags we should determine what the 5.x tags should look like and what to do with the existing tags (5.0, 5.1.0 etc)
What seems to make sense to me is:
5.x and 5.1.x, 5.2.x
In this way we maintain one tag for the Major version, and one tag for each minor version - patch releases wouldn't ever have their own tag.
In this way we can avoid tag bloat and keep the version tags manageable, meaningful and useful.
Currently we have a mish-mash of CiviCRM v5 tags on SE. 5.0, 5.0.0, 5.0.1 and 5.1.0 - Of these none of them are useful and I think this is contributing to their general lack of usage.
Ideally we should have one tag for each major version of CiviCRM (e.g. 5.x.x) or (v5) or whatever syntax people find useful. Fragmented tags for each minor version aren't manageable or really that useful. Major version tags help search and filtering out questions which pertain to older versions of CiviCRM.
As I spot questions which mention a CiviCRM version I try and tag these appropriately - however this usually results in me applying the 5.0 tag for any >= 5.0.0 CiviCRM version.
TL;DR: We at least need a generic 5.x.x tag - with the existing fragmented tags marked as synonyms unless there's a convincing rationale for keeping them as-is.