Creating new tags is strongly discouraged when they are redundant to existing tags or combinations of existing tags. They only cause clutter and confusion.
Oftentimes someone will make a new tag without realizing that another tag already serves the same purpose, and then new users will come along and, assuming that the newer tag is the correct one, use it instead of the original. Soon there are two different tags being used for the same concept, and the only way to find all matching images is to search for both tags. It's a serious problem that has bedeviled this site for years.
If you ever feel that the existing tag list is insufficient for tagging something you see in an image — an unlikely occurrence, given that there are literally thousands of general tags already in use — the best practice is to ask in the forum first, topic #12251 in particular. Chances are that someone else knows a tag or combination of tags that gets the job done without having to create a new tag.