API Design: When is idempotency a must-have?

Hey everyone, I've been thinking about API design lately. We all know idempotency is super important, especially for things that change stuff, like POST or PUT requests. But, like, how far do you take it? Are there certain situations or kinds of APIs where you just *have* to have idempotency from the get-go, even if it makes things more complicated? I'd love to hear your experiences and what works best for you.