Berachot 8.4 - The basis of "Mayim Aharonim" and holding like Bet Shamai vs bet Hillel

This Mishnah discusses the legal basis and origins of the custom to wash our hands after eating and before saying Birkat Hamazon. Otherwise known as "Mayim Aharonim" - after-waters. Is this still applicable today? What was the ancient custom of dipping one's fingers in salt at the end of the meal? How much of Jewish Law is based on healthy habits and staying away from physical danger?