Insecure people feel more comfortable approaching a person who is smiling because they seem nervous and vulnerable. I prefer a natural facial expression, at the very least never fake smile or exaggerate.
Pictures always look better without a smile. It doesn't make them look friendly at all but weird, even if its natural. I'm at the library right now and took a look around me at around 20 people without a single smile. A natural smile can't last more than a few seconds.