I think that monogamy is not assumed until you have gone out on a few dates. So i don't think you have to talk about poly until you have had a few dates. It is like how you don't have to talk about marriage, sexual desires or medical conditions until you get to know the person more.
However, if the topic comes up, I think you should talk about it. Also, if they may meet the people you are involved with, I think you should say something.
On the other hand, if you talk about it first, you may weed out someone who is truely not interested before you start to get involved. You just have to watch out for the reaction that if you are poly then you must be a slut.
As for how to come out, I think you should keep it simple and let them ask questions. Otherwise, you may sound like you are pushing the concept or you may push them into acting like they like it when they do not feel comfortable.