I was once told by a Japanese woman that in Japan one sneeze means that somebody is talking about you but two sneezes mean they're being mean about you. "So," she said, "If you sneeze twice it's better to sneeze three times." That was when I asked her what it meant if you sneeze four times. "It means you have a cold," she said.
Not that it matters what we say- saying "god bless you" after a sneeze is a colloquialism that has basically lost its original meaning, like "oh my god", or the fact that the days of the week are named for Norse gods.
I don't say anything just as I don't say anything when someone coughs, hiccups, belches, or passes gas. And I refuse to be a part of any continuation of ignorance-based superstitions (which I suppose is redundant).