I think the most important factor is how the texts are related to one another. Here are my (current) opinions on each of these categories:
#1 - Questions asking about perceived or actual contradictions within a single work written by a single author
I think these are great questions and they are on topic. The two texts cited are clearly connected being that they are now extant in the same work.
#2 - Questions asking about perceived or actual contradictions within a single work that may have been written by multiple authors and/or redactors
I think these are great questions and they are on topic. The two texts cited are clearly connected being that they are now extant in the same work, although one of the texts may or may not have been interpolated. For instance, do the creation accounts in Genesis 1 and 2 conflict? If you adopt the documentary hypothesis, they may have been written by different authors. If you argue for the unity of the work, then it may be a literary or rhetorical technique intended by the author.
#3 - Questions asking about perceived or actual contradictions between texts believed to have been written by the same author
I think these are great questions and they are on topic. The connection between the texts is clear: the same author purportedly wrote both of them (although answers are free to disagree with authorship of one or both of the texts).
#4 - Questions asking about perceived or actual contradictions between different texts written by different authors in the same time period
I think these questions are generally on topic, so long as the connection between the texts is made clear. For instance, the following textual connections are easily justifiable:
- The texts describe the same (specific) historical event(s)
- The texts prescribe a belief or practice to the same (specific) audience
This is not the to say that these are the only acceptable textual connections, only that these are generally the most obvious. Other textual connections should be explained, because underlying every connection is an assumption (whether that of authorship, context, etc.). The assumption should be stated if not obvious.
The connection should not be anachronistic to the original historical, literary, and/or linguistic context in which the text was written. For instance, asking whether a Psalm that appears to support the Christian doctrine of the Trinity conflicts with a Psalm that appears to oppose it is off topic. The very idea of a Trinity is anachronistic to both texts (such a question should be asked on a site such as Christianity.SE).
#5 - Questions asking about perceived or actual contradictions between different texts written by different authors in different time periods
The main example of this is when one of the texts is in the Hebrew Bible or Apocrypha and the other text is in the New Testament. The key thing is explaining the connection between the two texts. When focused on understanding the New Testament passage, these questions are usually good, when focused on the Hebrew Bible passage, these are often off topic (since the NT contradiction is likely anachronistic to the HB text).
If the texts purport to describe the same (specific) historical event(s) or a New Testament passage explicitly cites the Hebrew Bible or refers to it, the question may be on topic if the question starts from and is primarily focused on understanding the New Testament passage.
It is imperative when asking these questions that the textual connection be explicitly stated, whether it be a specific hermeneutic approach or a historical, literary, or linguistic reason for believing the texts are relevant to one another. These questions may be prime candidates for migration to Christianity.SE.