Using version-x tag styles don't add any value compared to use drupal-x. In my opinion, using the version-x tag is too generic, and is harder to understand, especially for outsiders.
If a random new user, see a question tagged version-x, he would need to think about what that version number means. Some modules like views have major releases, so what version are we talking about. Drupal also has minor releases, so if a user wanted to ask a question about Drupal 7.6, he could tag it both version-7 and version-6.
On the other hand, if you use the drupal style tags, it's quite clear what we are talking about. Anybody seeing a drupal-x tag, will know that this question is about or related to drupal 6. It's simpler and easier to understand.
Tags is a great source of filtering questions. Either by marking them as interesting or ignore. This value will be lost, if tags are made too generic, as they will stop being useful to filter questions.
We should use [version-x] tags. The reason behind this is that:
- drupal.SE is about Drupal. Differently from stackoverflow.com, questions that are not pertinent to Drupal are off-topic.
- The version reported in the tag is not only the Drupal version, but also the version of every module. There is just a module that doesn't follow the usual version schema, but that module is compatible with any version of Drupal; Drush 4 is not the version for Drupal 4, but Nodewords 6.x is the version for Drupal 6.
- We don't need compatibility with stackoverflow.com, as drupal is a tag that is used on SO, but not on drupal.SE. The compatibility is kept, anyway, as the drupal-6 is kept as synonym of version-6. Users who try to use drupal-6 would still see it, and questions using drupal-6 migrated from SO would be changed to use version-6.
- Tags like version-6 are less prone to be used when they are not needed. Users would probably choose drupal-6 on the basis that the question is about Drupal; this is not the case of tags like drupal-views, which should suggest the tag is for questions about Views.