For example should What is the Hermeneutical significance of the lack of direct mention of God in Esther? be tagged esther rather than (or in addition to) old-testament
Yes, esther for a question localized to the book, and old-testament only for questions that relate to the OT in general. It would be unusual but not impossible to tag both esther and old-testament, for example in this hypothetical question:
"What significance does the book of Esther have in the overall plot of the Old Testament"