Why not smile on a passport photo?
Smiling in US Passports is no longer a problem. As long as your smile is natural, and you do not scrunch up your eyes or exaggerate the expression, the photo should be accepted. However, many passport photographers are unaware of this, and will instruct the applicant not to smile.
The reason smiling in passport photos had previously been strongly discouraged or banned has to do with international security measures. Many modernized airports now use advanced biometric scanning devices which contain facial recognition software. Ideally, a targeted passenger's face can be scanned electronically and compared against a database of legally obtained passport photos. Distinctive biometric patterns, such as the distance between one's eyes or the shape of one's mouth, can rarely be sufficiently altered to prevent a match.
Some people prefer not to smile in their photos, as a blank faced photo is always accepted without problem. However, I personally smiled in mine, and showed teeth, and there was no problem. I received my passport in the mail within a few weeks, and the smiling picture was used.
I will post a link to my passport photo in the Discussion section.