We went through the fake smile school pics with Cub from 1st - 5th grade. Cubette, on the other hand, has the full, hearty laugh and smile. No holding back with her. It becomes contagious.
I don't care for grins, especially in writing. Seems sinister when read. Not necessarily sinister when seen in person where other facial aspects add to the interpretation, though.
I say, if you're gonna allow your feelings to show on your face, go all out and SMILE. Or, laugh even. You'll feel better and everyone else will, too. If eyes are the window to the soul, smiles are the window to the heart.