In my opinion, one of the most important pieces of information you can give me to help me solve your issue/challenge is the Sitecore version you are using. On SO, we already have a few version related tags, both for major and minor versions. Are we happy with those? Should they still say Sitecore8.1 or is v8.1 enough seeing as we're on the Sitecore StackExchange site? What about including the revision/update? Naming convention?
I've been removing a lot of Sitecore 8.1 tags earlier today, as none of the questions involved - in my opinion - were specific to the actual version.
I'm a bit worried we'll end up in a situation where questions get marked for 8.1 or 8.2 for instance, while being relevant to users of Sitecore 6 as well.
However - I also agree - the version matters; in many instances. But should it be a tag? or should it be something we ask to be clarified (when we feel it could be relevant) in the question text itself?
I'm leaning towards that. That we ask for clarification/update the question text - rather than it being a tag - since we can't really do a "sitecore-6.2-sitecore7.2" tag either.
But excellent question; let's get the ball rolling and see what people think :-)
Unless the question is about the subject of the version itself, tagging questions with version numbers is usually incidental to what is actually being asked. If the version number is relevant for getting the correct answer, that information should be included in the body of the post.
But I rarely find it useful to categorize questions broadly by version number — tags are for search. If knowing the version number is important to getting the correct answer, you definitely should not be using tags to provide important context.