Depends what mood i'm in to be honest.
I'll nod at bikes going the other direction
I'll nod at learner if they nod first and aren't riding like a ****
I'll nod at bikes going the same direction if i pull up beside them at lights/junc
I normally give people a flash or hand as i go by them if i'm following and go by (some people take this as a race invite though
so i kinda tend not to do it anymore)
The only people i won't nod at are those riding in little or no protection ( bar helmet) to whom i shake my head
But if they want to look like raw beefburger then thats their choice, i like my skin thanks.