I think you have to guage this based on your personal comfort level. I would not be comfortable telling someone who's not poly that I am poly. Mainly because I exist in a conservative environment and its really a matter of self-preservation. However, even if I lived in a non-judgemental community (if there is such a thing) I would probably still be hesitant.
Firstly - because what are the odds that they would be accepting of that? Probably not good.
Secondly - just when exactly would you tell someone that you're poly? No matter what point you are in the relationship, there's a strong possibility that its the wrong timing and you just come off looking like a fool.
Much easier to put the horse before the cart and make sure they're poly before contemplating a romantic relationship.
Take the risk of thinking for yourself, much more happiness, truth, beauty, and wisdom will come to you that way. - C. Hitchens
Me: Male, bi, 48, flexible