Question: Is it ever ok to tuck your napkin into the front of your shirt?
I love talking contemporary etiquette, but I get equally as excited (yes, I said it: excited) about the perennial, classic content. I happily perused my collection of vintage books (and their modern incarnations like this brilliant video) to dust off some topics that, while timeless, are still totally relevant.
Good table manners are one such topic. Good table manners are sexy. Jamming one's napkin down his or her collar, is not.
Is it ever OK to tuck your napkin into the front of your shirt? If you are eating a meal that you just ordered from the children's menu (and are of a suitable age for said menu), sure tuck away. Otherwise, no. Napkins stay on your lap -- if you have to leave the table, they go on the chair, not the table, as no one wants to see the physical evidence of what you've wiped off of your face.
The only other expectation to when it is OK to tuck a napkin into your shirt is if you are applying pressure to a wound. Which shouldn't be done at the table, anyways.