The etiquette is, do as you feel you should!
I am not a chronic greeter, but at the same time I don't flat out ignore people (they will get a nod or a raise of the hand), but I rarely initiate it. Especially if I am riding hard, in those circumstances I like to be left alone to concentrate, part of the reason I use the turbo so much is to remove any distraction and just focus (I don't run or ride with music, not even on the turbo, occasionally the tv if doing a long slow spin on the turbo, but never when riding at effort).
As for club's ignoring people, well the club I have been riding with the last couple of weeks do not ignore people!