i usually don't nod to anyone who doesn't do it first, except if i'm in an unfamiliar place or a bad neighborhood or at work. It's not out of acknowledgement of the other person it's more letting them know that i noticed them. I see it kinda like walking through grizzly bear country: you clap your hands and make noise so the bears know your coming and and don't get suprised, and it lets them know you're not scared and aren't good prey. At work it makes you seem more agressive and like a "team player." sp people notice you more.