I always encourage my tenants to introduce themselves to any new tenants moving in, especially new tenants moving in above or below them. That way, if people meet on good terms, they have a better chance of resolving issues between themselves without coming to me once the situation has boiled over. Often, by the time someone comes to me to complain about something another tenant is doing, there has been animosity between them for some time, and they won't communicate with each other. Starting off on a good foot helps keep the channels of communication open so that they'll at least talk to one another before they come to me to resolve the problem.
So, I'd say smile and say hello at the earliest opportunity. I wouldn't bring food or anything until they've had a chance to unpack and have dishes and cuttlery with which to eat it, but a smile and greeting is appropriate anytime.