I don't mind smiling, but one thing I never liked was how for school pictures, my mom would always insist I show my teeth - somehow, my smiles look so insincere and forced in those old school pictures trying to show as much of my teeth as possible.
On the other hand, depending on the person or the pictures, sometimes a lack of smile can make the person look so miserable . . . like Kristen Stewart, for example: it's rare that she smiles in her pictures (and she somehow figures if she does, she's "selling out" to her fans), but when she does, she's actually really cute, otherwise, she looks like she doesn't want to live on this planet.