Tags are meant to be freeform, there is no reason a content administrator can't come back and add relevant tags to a record at a later point, but as soon as you create a taxonomy of tags, you kill the ability for the system to evolve and rely on the administrator to keep up with the changing business.

You've just delineated one of the characteristics that my Russian-born wife (may God keep her close) said about the Soviet Union: if it belongs to everybody, it belongs to nobody. You MUST have at least one "owner", someone who will review the documents to make sure that the person listed as the Tier 3 support for a product still works for the company (not something you want to learn at 2:33AM during a Severity1 outage). If you can have the approved tags in a drop-down list or checklist, then you can make it easier for people to add tags, since they won't have to think about an appropriate tag, it will already be on the list.