I don't understand this reasoning. If there is a subtle reason wouldn't you take the time to think through it carefully? Are you in such a rush that you don't have time to decide the required data structure (ordered vs unordered)? Or do you have insufficient control of downstream software that you fear for unknown bugs caused by this? And since insertion order is often related to some other ordering in the input, you are comfortable that downstream software completely rely on this ordering even when it's undocumented? Genuinely curious because this kind of reasoning is alien to me.