I tend to agree with the second use (tag for typographical questions).
I don't see a great value in adding the serif/sans-serif tag to existing font-identification questions. I don't imagine people generally looking for serif + font-id in a search, as there really would be no value (font-id questions, in general, are not really that much visited unless they refer to a known brand, movie or similar). And the process of adding the tags to the existing questions would take a long time, as we don't want to bump all of them to the top.
I do like the idea of the tag as a descriptive discussion tag for typography. We don't have that many typography questions yet, but we certainly want to :) I see you have edited some existing ones, it makes complete sense, they look great.
But I don't really see anything negative with adding the tags to any type of question, so I'd love to hear some more opinions!