Sometimes I have added tags and looked to see how many blogs show up with those tags, then get rid of the ones where there are no other blogs listed. Is that not good to do that? Usually I only have 1 category and 5 or 6 tags.
It is not good to do that, trust me. Look FIRST; t structure of tag pages is identical, so just check http://wordpress.com/tag/whatever first. And there’s nothing wrong with using a unique tag once in a while.