How should I pick a flour volume to weight conversion for an arbitrary recipe?
This may seem to be an overly simplistic response, but if it's in a decent cookbook that deals with baking, the book often has an introduction or appendix or chapter that describes how to measure flour.  If it does, I measure the flour according to their directions, which can give a weight estimate.  Older cookbooks tend to advocate a lot of sifting before weighing (in my experience).  Newer ones sometimes mention spooning then leveling.  And I've seen at least a couple which just admit they scoop and sweep.

If the recipe is from a book without such instructions or an internet resource without such instructions, I generally avoid it unless it's a type of recipe where I know exact flour measurement is unlikely to have a big impact.  If I end up using such a recipe with no guidance on flour measurement, I sometimes try to take the era of the source into account if I know.  Otherwise, I'm not sure my personal practice matters, because I'm just guessing -- like anyone else who answers this question will likely be.  (Frankly, I worry this question is likely to turn into a poll.)