Tags are mainly used to 1) easily identify or find questions and answers on a common topic or 2) monitor specific subjects. In both cases, tags are naturally created once there's enough material to be tagged with (+1 for CG answer!).
Having that said, if we're not sure whether this tag should be created or not, it's probably because there isn't enough content @ PMSE over this specific topic or else the community itself would have already created it.
The only scenario where the above wouldn't apply that I could think of is when there are several questions with a common 'matter' that could become a tag but no one had noticed it before. For instance, assume we already have several questions that could be tagged as 'canvas' but no one ever mentioned that this was an actual technique. But I don't believe that's the case at the moment.