It is absolutely important to tag questions with denomination/sect, especially when the question focuses on a belief that may be unique to that group. This is essentially the same principle which applies to specifying Bible translation/version when posting quotations or references. Without this information, those who reply to your post can be very confused as to what your preconceptions of faith are, and cannot provide good and appropriate answers.
I think the analogy to RPG.SE isn't quite that far off. For those familiar, I think the comparison is more like this. (Note: By this analogy, I'm not intending to state that any particular group is "more right" or "more up-to-date" than another.)
- Let's say that, as a group, most Protestant Christian denominations are like D&D 4e.
- Roman Catholicism could be considered to be like D&D 4e Essentials.
It's based on the same core rule-set, but with some additional flair and
methodologies added through supplemental rule books.
- Mormonism on the other hand, is more like something along the lines
of Legends of Anglerre. There's a different set of rule-books, but
the world and objectives of the game are very similar.
There's going to be a good bit of overlap between 4e and Essentials, but there's some questions about Essentials that don't apply to all 4e Player Characters. However, a person who is only familiar with D&D can't hope to answer questions regarding Legends of Anglerre, except where those questions involve very general RPG principles and concepts.
I myself am not familiar at all with Legends of Anglerre, except that it is another RPG set in a fantasy world similar to those in D&D. (Thanks to @CRoss for the reference.) I'm also not deeply familiar with Roman Catholicism or Mormonism in particular. So, please forgive me if the above analogy is somewhat crude and inaccurate in some regards. I hope the proper message has still come across, and applies well.
That said, there are still some questions that can be considered "system-agnostic" among Christian faiths. Although the "christianity-agnostic" tag would be amusing to see around, I suggest using the more widely recognized term, "non-denominational".