The purpose of a tag is to provide a useful set of questions to make it easier for (potential) question askers to find the info they need and/or make it easier for experts of a specific subject to find things to answer.
The general standard should be something along the line of "does the subtag refer to a specific subject that may generate questions of a different nature than the general tag." If so, the tag should exist if there are a significant number of such questions. Otherwise, they should be synonyms.
If both tags are useful (and there are many cases of this type), then the more specific one should always be used when it applies. The more general one can be used, but has a low priority Thus, if there are 5+ other tags that could apply to a given question, the more general one should fall out first.
On the specific examples:
I doubt the number of non-Bible-text-to-English-translation questions are very common. As such, bible-translation should be merged with translation. If a distinction was really needed, it should be a complete split ala history vs church-history.
premillennialism is a specific teaching that generates its own class of questions. It is OK if such questions are also tagged eschatology - people interested in eschatology in general will sometimes be interested in these questions, while other people may well be interested in only premillennialism questions.
The last example is the most difficult. I tend to think of the sermon-on-the-mount as being distinct because of the moral teachings it offers, but would not be opposed to merging it with words-of-jesus. I would use sermon-on-the-mount for asking about the moral teaching/doctrinal issues type questions, and words-of-jesus for questions asking "what does Jesus mean by X". If the question impacts both, use both tags, but don't automatically use both. I would only use gospel-of-matthew if the question both quotes the text and asks an exegesis type question. (Although not mentioned in the OP, jesus could technically apply. I don't recommend using that tag when any more specific one applies unless the question is also asking about the nature/character/etc. of Jesus himself.)