First, we all know that greek and roman mythology is so similar that "greco-roman" is a legitimate term in all academic circles, so perhaps greek and roman tags should be synonyms of a greco-roman tag.
But, there seems to be a convention developing where a good portion of tagging revolves around identifying the geographic location where a myth was prominent. For example, there is pacific-northwest, chinese, and irish, all in use to identify that the question is about myths from those areas.
So it seems like having a greek tag and a roman tag is a necessity.
The question now: Should we have a greco-roman tag, or should questions just use both greek and roman as needed and greco-roman should be a synonym of both?