This is a fun question. Personally, I like using "not bad" when I mean "good", but don't want to sound too enthusiastic or nothing very exciting has recently happened. When I say "I'm not bad" I am probably at around 80-90%. When I use "I'm not too bad" it means something might be bothering me a little bit, but mostly things are fine, so maybe around 70%. I think whether you use these phrases probably depends a lot on your personality and where you're from.

I use these very often. If I drink a beer that I have never tried, and the taste is pleasing, I will say "Hmmmm. Not bad." But also, if I am worried about a stressful day or a negative experience, and the actual event is better than I expected, I will say "Ok, that was not too bad." So, I use these phrases for both good and bad things.

Ha Ha, In Minnesota "not too bad" is a standard answer for "how are you?" But this can really vary. I would say in general, "not bad" means things are better than "not too bad." But this is very individual.

"Not bad" is usually better than "not too bad". It's also important to know that in our culture, asking somebody how they're doing, at least in public settings, is often more about saying hello than actually wondering how they're doing, which is not the best but is how things are.