From what I understand, use blunt when talking to commoners and use polite when talking to nobles.

I'm guessing all the people walking around and the shopkeepers are commoners. And the people in castles are nobles, but when do you ever have to ask them for directions?

Unless some nobles are in the taverns. If so, how would I tell them apart?

Or is there more to etiquette and streetwise than I think?

Someone correct me if I'm wrong but if I recall correctly, Etiquette skill is used when you are in court, plead Not Guilty and lie to defend your cause. Streetwise is used when you use "Debate" to defend your cause.

Some commoners do not like being talked with blunt and will only respon to normal. There are some occasions when Etiquette works on the people on the street. As you mentioned, NPCs inside locations are mostly useless as you cannot get directions from them. (They'll never point a location on your map). They're only good for rumors.

I never pay attention to streetwise and etiquette. In my experience if you have a high enough Personality, everything works out. Although, I've never had a high streetwise or etiquette skill, so I can't really say exactly how useful they are.

All I know is that many commoners won't help you if you're "polite" or "normal" in talking to them...they'll say "why would I help you" or "you don't have to talk down at me" or something. But if you use "blunt" a lot of the time they'll help. I usually go with "polite" "normal" or "blunt" depending on how they greet me. If they say something brusque like "What is it" or "What do you want" then I use blunt, if they're polite like "how can I help you" then I use normal. I only use polite when talking to people in castles. It's worked quite well so far and I have a pretty average Personality score.