I generally say, "I'm fine" if I AM fine or if I REALLY just don't want to discuss what's going on in my head (which is rare) and I'm not ready to even discuss the fact that there is something going on in my head. For instance if I'm really pissed off over something, but feel like I'm mayyybe being irrational/unreasonable about it. It's not a good time for me to bring that up, even by saying, "I'm feeling frustrated with you about something" or whatever. Especially with my kids.
But in general, I agree with everything you have to say here.
Thank you very much @byn. I totally get where you're coming from with the pissed off thing. Sometimes I do it too as I'm not sure yet if my anger is justified or if I'm just being a bit too sensitive or extra moody that day.