In the tag wiki for 7 there is a comment at the bottom:
Version tags should be used only when strictly necessary, and not used just to report "I am using Drupal 7 in my site."
While I understand the probable intention (not clogging up these tags), I actually find having the version number as a tag very useful when answering questions, if only to know what version the user is struggling with (and so avoid a comment to ask). Users don't always put this information in their title/question.
Furthermore marking each question with the version of Drupal it relates creates a natural grouping for the questions, making it easy for experts in the different versions to spend their time in the right tag.
I can't think of a good reason to not mark each question with the Drupal version, unless of course that question is not version-specific.
I may well be missing the point though, can anyone comment?