I've always thought that tags are a very important part of keeping the content on CV well organized. If there is a tag that really should exist to facilitate that task, you should create it. That said, @Glen-b's point resonates here: if there aren't any questions whose content needs to be indexed by a given tag, there isn't really any need for that tag to exist (yet).
Also, be aware that it a tag exists on only one thread for a certain period of time (I think it's six months), the SE system will automatically delete it. The idea is the same: there isn't really a set of information related to that topic that needs to be held together by the tag. Please don't add it to extra threads just to make the tag stick, but search around, there are probably more relevant threads that could be connected. If there aren't, the tag really should go; it can always be re-created in the future when it does become needed.