You have a Document Center and you want to use the Metadata Navigation pane. It makes sense to use a kind of termset structure to find easier documents related to a particular term and child-terms. Using the Metadata Navigation is a powerful feature but has also a limit because of performance optimization.
General capacity and performance information when using Metadata across the SharePoint farm is documented in the following TechNet article.
Estimate capacity and performance for enterprise managed metadata in SharePoint Server 2010 http://technet.microsoft.com/en-us/library/gg681889.aspx
Screenshot with a Document Center and Metadata Navigation enabled. Please apologize that the amount of countries is small and does not contain all possible values.
On the left side you see that I clicked on EMEA and all terms underneath of the EMEA term will be included into the Filter. As a result all documents on the right side containing a name of an EMEA city will be shown.
On the right side I clicked again on the button and the filter contains only documents with the term “EMEA”.
In case the amount of child-terms underneath of EMEA has more than 300 items, the filter will only allow to see the result on the right screenshot. Please have this in your mind when you plan to implement such a Metadata Navigation Filter in your Document Center.