My vote is "Yes"
I'm a supporter of tagging a single denomination as being sufficient for scoping a question to a denomination.
Re: Tags, from the tour:
All questions are tagged with their subject areas. Each can have up to
5 tags, since a question might be related to several subjects.
I interpret this as meaning that attaching denomination tag to a question properly places it within it's subject area, and thus sufficiently scopes it.
As the OP mentioned, tagging being used to scope questions is an increasingly frequent behaviour, that is because tagging is something that the average new user coming form twitter or youtube already understands. In my opinion the rising community has already decided that tagging a question with a specific denomination is adequate.
Christianity - Stack Exchange is a question and answer site for
committed Christians, experts in Christianity and those interested in
Nothing in that site description suggests that it is our responsibility to hand-hold new users. They can can read through the tour and learn how to properly use the site just as easily as everyone else, no community consensus is going to change the behaviour of people who don't bother to learn the site rules anyways, you're always going to have new users asking off-topic questions that are going to have to learn through downvotes and close votes how to use the site.
I don't see why redundancy should be required in questions. As a user from a denomination of unorthodox Christianity I'm constantly asked to specify my denomination not only in the tags, but in the question title, and the body of the question. Meanwhile, there are multitudes of questions that are considered on-topic, even though they are not scoped to preclude or exclude the views of certain denominations whose answer to the question differs from the majority view. Those from the minority who then offer an answer from their differing view are often downvoted, and the popularity contest ensues.
It's not difficult to check the tags when reading a question. They are shown in the listings on the questions pages, you can easily identify your favorite tags from the list, and you can ignore questions tagged with subject areas your not interested in or not expert in offering answers too. I have several tags that I ignore because their conception is heretical and conflicts with everything I know about God, so I don't bother reading them or offering answers anymore. It's my experience that most of the time when I'm asked to "better scope" my questions, it's usually from users whose view I know conflicts with doctrine I'm asking about.