The current state is to allow version tags and I agree with that. However, I would suggest to avoid versioning deeper that the major release.
The reason is the following.
The problem is that questions tagged with a version tag are, in the majority of cases, mis-tagged. Users working with a 3.x version tend to tag the question with any 3.x tag available even if, in most cases, the version tag is completely irrelevant.
There are very little differences between ruby-on-rails-3.1 and ruby-on-rails-3.2. Of course, a greater release can contain features not available in the previous minor version, but in terms of answer to a question, the solution is never drastically different. This is not true for major versions.
Moreover, the current configuration leads to misinterpretation of the major version tag. For instance, is ruby-on-rails-3 the tag for 3.x questions or for the 3.0? I tend to consider it for 3.x questions, but then it means we're missing ruby-on-rails-3.0. Vice-versa, if we consider it for 3.0 discussions then we are missing the global 3.x tag.
In fact, if we decide that 3.1 and 3.2 versions are so different to justify a tag, it means that also 3.0 is different than 3.1.
Before opening this thread I researched other framework tags, such as django and asp.net-mvc, but I don't have enough experience with them to make an effective comparison. I know, for example, that the development cycle of django is very different than ruby-on-rails and the tags django-1.4, django-1.5, django-1.6 probably makes sense because django minor versions are almost major releases (they have been releasing one minor release per year since 1.0 and they are substantially different).
Thus please consider this thread a proposal specific for Ruby on Rails versions.