Maybe it is just a matter of psychology, but I can't stand going periods of time without seeing someone smile or without smiling myself. I have depression, and just seeing other people smile makes me feel good, so I'll go out of my way to make everyone else happy just for the smiles. I once watched a movie called Wristcutters: A Love Story, and in this movie anyone who kills themselves ends up in an afterlife much like Earth, but worse. There are no stars, no flowers, and nobody is physically able to smile. Not seeing a smile just for the time through the movie made me so depressed.
I have quite a bit of flesh on my face for someone pre-T, and it really shows when I smile. But I look just like my dad when I smile. Whether or not it impairs my ability to pass, I'm not going to stop smiling just to like a tiny bit more masculine. If you already look pretty male, smiling isn't going to be the thing that gives you away.