If I know you or at least see you frequently a "hello" is appropriate. Otherwise you're some strange person I don't know talking to me for some reason I can't fathom, and probably want something. I usually wear sunglasses anyway so I can just pretend I didn't notice you. People who ask "how are you" I tend to ignore. If they press they get a brutally honest answer usually ending with "and also this person I don't know is insistent about hearing my personal business." It's not that I'm a rude person. I'll hold a door for someone before I'd let it slam in their face, I help people who have car trouble, but a stranger approaching me out of the blue just reeks of having an ulterior motive.