Not American (British), and it's never felt aggressive/threatening, but smiling in a way that shows my teeth has always felt exceptionally forced. It doesn't stand out as strange when other people do it, but it just doesn't feel like a natural reaction for me.
When you think about anything it becomes weird and deserves smile. Having said that I think since everything is weird then you can say that almost nothing is weird. And the key is not to think about it too seriously.
