Yea, I would argue that Atheists don't really preach, at least not commonly.
We don't have to convince anyone of anything. The same way we don't have to convince people that 2+2 = 4.
Its all the people out there that are taking a more wild position (like 2+2=5) that have to preach, proselytize, persuade, and persecute to advertise their worldview.
RE: If Darwinism is true, Atheists shouldnt preach!