I am guessing that smiling would be associated with EF types. INTJs aren't renowned for beiong natural smilers...
Mhm, EF types probably smile the most on average, with IT types being the opposite.
It's not really "out of politeness" though. I think, for many EFs, smiling and having a warm/inviting exterior is just built into our system. Of course some might do this - there are definitely EFs who now how to cheese it up for the crowd - but I'd theorize that it's more IFJs who do this. Fe in IFJs feels a lot more cool, polite, and guarded than Fe in EFJs.
People have also theorized about instinct variants and facial expressions. From what I remember it went like this:
sx/so - intense expression directed outward
so/sx - bright, smiley, intense expression
so/sp - smiley, calm expression
sp/so - withdrawn, soft expression
sp/sx - withdrawn, self-absorbed, calm expression
sx/sp - intense, self-absorbed expression
I kind of buy it and I kind of don't. so/sx definitely rings true for all the so/sxs I know, both on this forum and IRL.