Given the changes in 2.8, should we ask the users, whether they are asking or answering, to indicate which version they are using, in order to make sure that the answers provided are correct?
Or, alternatively, create versions tags 2.79 and 2.8?
It's not necessary. On the one hand, if the users are experiencing an issue, we would usually ask for screenshots, which makes it very clear whether they are on 2.7x or 2.80.
On the other hand, if they are looking for a feature or requesting advice on a workflow, the good answers will spontaneously start to transition during the Beta period from a majority of 2.7x-based to a majority of 2.8-based (and will be mostly independent from the version, apart from the hotkeys). They will likely contain the necessary remarks about "if you're using 2.8, you will find it in this other menu / with this other hotkey", either straight away or after an edit/comment by another user.
Of course, if the question is about fast PBR visualisation, or advanced creative uses of the Grease Pencil, the answerer will very likely push for 2.8.
I'm also against version tags. What the users need are answers about how to use Blender, regardless of version. A question is rarely about a particular version of blender: answers may be. So the solution is to slowly edit the answers we come across to be up-to-date.