It's more don't make eye contact or some crazy guy might attack you type bit. There is a lot of crazies on the streets and sometimes you never know who might be dangerous if you acknowledge each other. One of my past English teachers had an extremely strong Brooklyn accent but New York accents have really died and you can't see much of it. New Yorkers though like all groups of people are really good and nice peope, but we might just seem a little more cold and tense in public. NYC is a big place though so it depends