I dont know the rules for this one BUT!
- if I'm driving another car I have accidentally waved... and received the "who the, are you?" look, so normally no I don't.
- If standing on the footpath, I sometimes wave but normally that just ends in an awkward moment too...
On the flip side, if I'm the one driving and someone on the footpath/nondefender waves to me then I will wave back just in case. :)

