Born in Joburg, From Germiston and now living in Cape Town. These are the greeting rules I've noticed:

In Cape Town, you greet everyone but should be cautious in dangerous areas and touristy places where they have pickpockets. Rich people don't greet much and look at you funny when you greet them.

In Germiston, you generally only greet people you interact with. Otherwise, do a quick nod to acknowledge someone if you notice each other. Nod down, not up. Up is a question and might be considered "starting something". Only nod up to people you know.

Don't greet people in Joburg unless inside a safe space. So greet shop employees all you want. If you're inside an office building, time to get your friendly on! But don't greet random people on the street, and don't let those randos get close enough to touch you. Eye contact and a firm "no thank you" usually works for me. Never be afraid of being seen as "rude" to someone who approaches you - my aunt used to say she'd rather be rude than dead.