Out of the sixteen flats round here I only send a card to one flat and that's because the occupant sends me one. It's a bit awkward as we seem to have developed a pattern where the sender gets a card back from me, then the following year I send one then get one back from the tenant.
The thing is I've lost track and can't remember who sent one first last year! My thoughts are that if it was the other tenant then I'll have sent her one back, but If I just throw caution to the wind this year and send her a card before she sends one to me she might take offence, thinking "why's he sent me one, before I've sent him one"?! Then she might not send one in return. This is very awkward and it could lead to her not speaking to me for the whole of next year! The etiquette of card sending is a serious business and getting it wrong can lead to a whole lot of trouble!!!