Good grief.   There really are people who get affronted because they get a nod or a wave or a thumbs up or a touch of the cap or whatever, instead of a spoken response? And who expect everyone else to fall in with their hang ups or be taken as being guilty of discourtesy, whatever the actual or obvious intent?

Spare me such people.

As for Allen C finding something odd about a nod which starts with an upward movement of the head, but nothing odd about one that starts with a downward movement, that distinction seems pretty odd to me. Imagine making the same distinction when it came to people shaking their head, according to whether the gesture started with a left turn or a right return. (I've just tried that, and I think that an initial right turn is my normal preference.)

I work on the principle that unless someone indicates pretty unmistakably that they are feeling hostile towards me, anything they do is to be construed as friendly. (And even if they are hostile, a friendly greeting is a much better way of getting under their skin than a counter-hostile one.)