If there is only one instance of a tag, make it so people don't waste their time clicking it.
It's great to be able to add a lot of tags for each post, but if there is only one post with a given tag, then it should be non-clickable. This way visitors are more likely to click tags since they know that there will be other posts with the same tag.
Maybe in the TAGS widget it is OK to have orphan tags clickable, since you may not actually be viewing the only post with a given tag.
However, when displayed with posts (and in search results), it would be very beneficial to make these so they aren't actually links.