Show only tags in use
Where tags are currently shown in top of the screen - just below the colored top bar - all available tags are shown.
I suggest only tags in use are being shown. That way i don't accidently filter by an unused tag thus helping me keep my overview.
John Smith commented
If you add a Tag to a task and then delete it, you will have added a Tag to the master list. This isn't a huge problem as you can always delete it again very easily.
However if you mean "only show tags that are present in the Area you have selected" that would get my vote as it's probably not too hard to code.