Basically, as the title says.
I think people should be made aware that if they don't tag their questions then they expose themselves to particularly controversial answers.
I tried to make the site policies accessible by writing FAQ index (summary of site policies) -- which mentions using tags.
If you want to make people aware, you can -- the only way I know to do that might be to post a comment under the question, maybe referencing a meta-topic, and IMO preferably phrased as offering information (e.g. "If you want X, you can Y"), or phrased as a question ("Did you want X? Because if so..."), instead of giving orders or making demands or criticising (e.g. "You ought to..."), to be welcoming.
Sometimes I ask if they want answers from a specific school, and sometimes they reply they're open to answers from any school.
I think people should be made aware that if they don't tag their questions then they expose themselves to particularly controversial answers.
Remember, it isn't for moderators to police whether an answer is orthodox or correct -- that's for users to vote on.
As moderator I might delete an answer which "doesn't answer the question" if (only if) it's downvoted -- or if I can tell from the nouns it's using that it's objectively off-topic, like if it's posting about "Jesus" or something.
Maybe the most useful tag for what you want would be reference-request -- that tag requires that every answer must be, at least, backed by references. In that case a moderator can see "objectively" whether an answer has references -- a hyperlink or citation and a block-quote -- and delete it if not.
That won't guarantee that you agree but at least exposes some basis for the answer -- it may help the reader assess whether the answer is a "good" one -- and, perhaps even more usefully, provide references which the reader can use to read more of the subject themselves.
If the OP wants answers specific to the Theravada tradition, then he or she has to use the Theravada tag.
If an answer can even be narrowly or marginally interpreted as being related to the tags, then it will not be deleted by the moderators.
If a user feels that such an answer is technically wrong or inaccurate, then he or she should downvote it. Please see this answer for references to SE policies on this.
For e.g. we had one answer which argued that anicca meant "insatiable" rather than "impermanent", but this answer was downvoted and not deleted, because it is at least marginally related to Theravada, but the majority considered it to be wrong or inaccurate or unorthodox.
On the other hand, a hypothetical answer that states "there's no such being as the Manjushri Bodhisattva according to the Pali Canon", for a question on Manjushri which is tagged Mahayana or Tibetan-Buddhism, would be clearly off-topic to this tag, and may be deleted by the moderators.
This moderation policy is different to SuttaCentral, which considers "Anicca doesn’t mean impermanence" to be an extremist view that may cause the user to be banned. On BSE so far, we do not have a policy for moderators to judge whether an answer is technically correct or accurate or orthodox. This is to be judged by the community using votes.