Only in UK cities. In villages and hamlets almost everyone says Hello to everyone, even if they don't know them. This also extends to drivers raising their hand from the steering wheel in a little greeting when passing in country lanes.
> (Also: the American practice of baring your teeth when smiling is often received as aggression.)
This is patently ridiculous.
American here, I have always found it disingenuous and a little creepy when people smile while baring all of their teeth. There is a difference from showing some when extremely happy and the practiced facade people selling something have learned to do unconsciously, notably when lower lip curls down for more dental exposure.
Brings to mind the smiling with upper and lower teeth sketch from Silicon Valley
It doesn't mean they're not nice people: just that by starting an interaction trivially you've invaded their comfort zone.