It never makes sense. If you know the site already, the question isn't necessary. If you have to ask the question, the site tag is meta-tagging. It's also not a question about that site either, it's a question about the network as a whole.
--Nij
Tags should be used only taking into consideration the information in the question. That's the same reason we don't use tags about the (potential) solutions on a question that doesn't refer to the solution.
