Hi, I just ran across this post while searching for something. There have been a few threads on this topic that you may find interesting:
When Did People Start Smiling in Photographs? featuring /u/Axon350
When did smiling in photographs become common? - more from /u/Axon350
When showing teeth (in art, photograph and portraits) became culturally acceptable? And why it wasn't before? - /u/GeeJo gives a quick survey of smiling in art