I don't say hi until I've had time to observe the local culture.
In NW Arkansas/SW Missouri, what was proper was a big smile and an enthusiastic wave to go along with some variation on an effusive "Hi!!" That was the standard of basic politeness, it's what you did if you didn't want to send an "I hate you, you're beneath me, suck off" message.
I had a hard time with it, but eventually realized I liked it. It was a good thick social armor.
If I couldn't smile, I stayed home.
In rural northcentral West Virginia, one should make very brief "area-of-the-eyes" contact (I think ASD runs particularly thick up there, b/c it was long after I left that anyone said anything about eye contact, and I never knew it wasn't normal to not look directly into someone's eyes) and say a casual "Hi" (usually ends up as "H'lo" or "mornin',"lift a hand or nod.
In Morgantown or Fairmont WV or the greater Pittsburgh area, one does not speak, wave, smile, look at the person, or say anything along the lines of "Hello" or "Good morning" or anything else unless you have seen each other often or been previously introduced. It seems to frighten them, or for some other reason to provoke hostility, if you're too darn friendly. I hated it about Morgantown/Fairmont as a young adult, and as a middle-aged SAHM I find it really isolating now. I work at making cracks in it though. If I'm open to communication, I will very quickly lift one corner of my mouth, or barely nod, or say "You're welcome" in response to a cashier's perfunctory "Thank you." After many repetitions of this, pleasantries may be exchanged. And there are bored, lonely, or otherwise outgoing people with whom the rule does not apply. The trick is just finding them.
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"Alas, our dried voices when we whisper together are quiet and meaningless, as wind in dry grass, or rats' feet over broken glass in our dry cellar." --TS Eliot, "The Hollow Men"