Right now tags are specific to boards and not a global item. In other words if I have 3 boards all with the same tag than the system creates 3 different tags. While the Search Everything will still work and show you everything from all 3 boards that have that tag the dashboard widgets cannot handle grouping the data from those 3 different tags together properly. You also end up with a very cluttered tag setup where you may have the exact same tag multiple times. Considering the purpose of the tags I would think globalizing them would make the most sense so you can then use the same tag across multiple boards and your dashboard would know how to handle it.
Attached are a couple of screenshots showing you the problems this creates. The first is just a cluttered tag list with multiple of the same tag. The second is a pie chart on the dashboard with the same tag showing as separate pieces of the pie when we really need the same tag to show as one piece.