Anyway -- I regard most of those warnings as quite overblown. I mean,
on lots of subway cars you stand out more if you don't have white
earbuds in, probably attached to iPhones. Midtown is very safe. Your
laptop bag doesn't have to say "laptop" on it to be recognized as such,
but there are so many other people with laptop bags that you won't stand
out if you have one. Subway crime? The average daily ridership is
about 5,000,000; there are on average 9 felonies a day on the whole
system. To quote a city police official I met, that makes the subways
by far the safest city in the world.
That's probably an abuse of statistics.
Yes, you're probably at more risk if you look like a tourist. But there
are lots of ways to do that, like waiting for a "walk" sign before
crossing the street... (Visiting Tokyo last month was quite a shock to
my system; I had to unlearn all sorts of things.)
Looking and acting like you belong is good advice in most circumstances.
Act like the other monkeys. If you don't give someone reason to question
you, they probably won't. Wait, oh, that's the guide book for infiltrating
facilities ... 
... JG