Since Magento 1.9 is released it's better to manage related tags for this new version. So the people will more enthusiastic for asking more about Magento 1.9. This is just my suggetion. Feel free to discuss :)
2 Answers
If you're gonna start tagging by version, you should create tags for most/all versions. But I think that's something you might want to hold off on until 2.0