As of mid 2013, the wiki tags indicate that osx-server is only for Snow Leopard and older server installs. I have been retagging server.app to all Lion and newer questions and removing the osx-server tags when it's not used on 10.6 and earlier.
Perhaps we will want to re-evaluate this and make a synonym to point them all to the same tag and/or re-tag old questions.
The only problem is that old server installs aren't served well by server.app but the number of people that need that distinction will shrink rapidly. Also, I wonder if we can prevent use of the old tag and still link things so the new tag feeds both in terms of badge counts and searching.