Version tags should be used only when the question is specific for a Drupal version, as they don't mean "I am using Drupal X." To make a comparison with a programming language, using 7 for a question that is valid for Drupal 5 and Drupal 6 too is like using c#-4.0 for a question about the
if control statement in C#, which didn't change in C# 4.
Using a version tag, you are saying to the users who read the question that it doesn't interest them if the are using a previous version.
There is also a problem with a tag used with most of the questions as that tag would stop to be useful. Imagine having drupal as tag: Since it would be used on every question, it would not classify any question, which is the purpose of tags.
If the user doesn't say which Drupal version is using, the answers could answer for both the current version, and the previous version. This is preferable, and it would avoid the same question is once asked for a Drupal version, and then for another Drupal version.
Last, the Drupal version can be given in the question text.