Jaynek,
I've always been told that we are to understand scripture literally UNLESS the Church has said this or that passage is symbolic, metaphorical or prophecy. Of course, we can't know that unless we read scripture with a concordance, which gives scriptural commentary.
I agree that it is very important to read Scripture with the mind of the Church and that using approved commentary can help us to do that. Pope Leo mentions the issue of understanding Scripture literally in Providentissimus Deus (15) where he exhorts us to observe "
the rule so wisely laid down by St. Augustine-not to depart from the literal and obvious sense, except only where reason makes it untenable or necessity requires." This suggests that we ought to be applying reason to what we read to see whether a literal interpretation makes sense.
For example, consider these words of Our Lord (John 15:4,5):
Abide in me, and I in you. As the branch cannot bear fruit of itself, unless it abide in the vine, so neither can you, unless you abide in me. I am the vine: you the branches: he that abideth in me, and I in him, the same beareth much fruit: for without me you can do nothing.
It does not make sense to say that our Lord is literally a vine. I think we are permitted to consider this a figure of speech without an explicit teaching from the Church telling us that it is. Of course, whenever we draw such a conclusion we need to be prepared to change if we discover the Church teaching otherwise.
In the case of the flat earth issue we do have Church teaching to guide us. Pope Leo XIII taught us that
Scripture is not intended to teach "the essential nature of the things of the visible universe, things in no way profitable unto salvation" and that such things are "described and dealt with things in more or less figurative language, or in terms which were commonly used at the time". So we are definitely justified in interpreting Scripture that way.
There is also Pius X's teaching, specifically concerning Genesis, that "
it was not the intention of the sacred author, when writing the first chapter of Genesis, to teach us in a scientific manner the innermost nature of visible things and the complete order of creation but rather to hand on to his people a popular account, such as the common parlance of that age allowed, adapted to the senses and to man’s capacity" so it is not necessary to treat it as if it were scientific information.