Mine is mostly this. I have been criticised by strangers for my normal/blank expression being sullen or hostile (apparently “no expression” = “grumpy jerk”), so I’ve developed a kind of tic about smiling if I accidentally make eye contact with someone. This being London, though, that doesn’t happen very often (eye contact).
Yes - I smile at strangers and often nod or exchange a quick howyadoin. I also initiate small talk if appropriate, and always talk and act pleasant to shop keepers and such, including please and thank yous.
It just makes the world a little shinier, IMO. I know that when we pass in the street, and I give a small smile and you immediately look away with a sour face, I think you’re an asshole.
All the time. And I immediately have a little bit of distrust for people who can’t be arsed to do at least this…in my community. I don’t expect people to smile at me in NYC, for example, but in my town, it’s just polite. And I hate service people who don’t make eye contact and don’t at least smile. It’s 2 minutes out of your time, and I’ve done your job, and always managed to remain cheerful and happy.
Never. It only encourages them to talk to you.
I smile at strangers all the time. It costs me nothing and adds a little joy in their lives (maybe). Most smile back, some are surprised that I even noticed their existence in a positive manner.
male.
I smile. It costs me nothing & it might make someone’s day.
With some people thats right, but there are a few painfully shy people too. Just sayin’.
You know, I wish somebody could’ve told me this when I WAS EIGHTEEN!!!@@#!!#!!
Er… a-hem! Sorry 'bout the outburst. 
Smile, nod, maybe say howdy. Why not? It’s nice to be nice.
Usually. I live in NE and everyone is just so darn nice here that if I don’t, I feel like a grumposaurus. That said, I never smile at anyone in the airport because I am a grumposaurus whenever I’m thre.
I always smile at anyone I make eye contact with, it’s automatic. Hell, sometimes I’ll make eye contact and smile at someone before I realize that I, in fact, actually dislike that person.
Damn reflexes.
I smile at people I’m planning to cheat, con, or otherwise do ill to. I find it makes them easier targets.
Female. Northern Virginia. I smile at everyone… and even say hi, to most stangers. I make a notable effort to walk with my eyes up and back straight. As such, I do make eye contact with most folks as I go. So a smile and a nod just seems right to me.
Smile rapist. 
Male, and I smile at people when I make eye contact. I usually will throw out a “Good morning”, “Good afternoon” or “Good Evening” too.
Male, Southwestern Virginia. I nod and greet anyone I make eye contact with. Sometimes I smile, but I don’t do it on purpose. If I don’t speak I will always smile during the nod, though.
Female, Southern, lives in a small town, works in the hospitality industry: You may not be surprised that I smile at everyone, often greet total strangers, and make small talk with random people (in line at the grocery store, in waiting rooms, etc.)
This is a joke, right? I honestly can’t tell considering your ummm… prior history.
As a general rule I make brief eye contact and smile at strangers. I feel it’s the polite thing to do. I’ve always done it, and many people smile back. If my presence is acknowledged in any fashion, I’ll typically follow up with a small head-nod. Maybe it weirds some people out, but I think most people appreciate it.
Of course, now i live in Japan, so it complicates things even more. Greetings are considered very polite here (in fact passing somebody without any greeting is very rude, although common.) Still, eye contact and smiling are not really part of that social requirement. I still do it though. If I was Japanese it would probably be off-putting, but as a big foreigner most people seem to like it.
Being a white male in a big city smiling at strangers would just get me in trouble, so no.
Change “hospitality” to “nursing,” and you’d have my post. I even wave to people in cars.