Fortunately I haven't run into this all that often. My rules on it
- Guidance can't be declared after the fact, once a D20 is rolled that's it.
- Guidance can't be cast if the caster has no reason to know that it's needed. If a player asks for guidance, their PC is as well; the PCs say whatever the players say.
- It's not particularly useful in social situations, if an NPC sees someone casting a spell they're going to assume the worst and become one step more hostile.
- If combat is a possibility casting any spell that can be detected by your opponents is considered a hostile act. Roll for initiative.
So some simple logic tends to keep it to a reasonable number. Or I've just been lucky and it hasn't been an issue.