This is the sort of thing you will simply learn with experience. As you say, it's mostly dependent on the meta and whether there are important Pokemon you could hit using the opposite stat that you couldn't with your main stat. You will become a better judge of which targets are important, and which moves are worth sacrificing for, as you familiarise yourself with the game. In most cases, though, mixed sets are a liability to offensive Pokemon and require more justification than usual.