Every grain of food and every drop of water (or other halaal drink) is a great bounty of Allah Ta‘ala. Deliberately wasting any food is indeed a severe ingratitude. Ingratitude to the favours of Allah Ta‘ala results in the wrath of Allah Ta‘ala descending and the snatching away of that bounty.
The practice of Rasulullah (sallallahu ‘alaihi wasallam) in this regard was to thoroughly clean the utensil from which he ate. He also stressed upon the Ummah to do so. Jaabir (radhiyallahu ‘anhu) reports that Rasulullah (sallallahu ‘alaihi wasallam) instructed that the fingers should be licked after eating and the utensil should also be cleaned. Rasulullah (sallallahu ‘alaihi wasallam) then said: “You do not know in which particle of food the barakah (blessing) lies”. (Saheeh Muslim #5300)
Furthermore, Rasulullah (sallallahu ‘alaihi wasallam) is reported to have said: “The one who eats from a utensil and cleans it completely (so that no particle of food is left), the utensil supplicates for forgiveness on his behalf”. (Tirmizi #1804)
It is therefore clear that the so called “etiquette” of the West of deliberately leaving over some portion of the food and drink in the utensil is totally against Islamic etiquette. It is a sign of ingratitude to Allah Ta‘ala. Thus, the followers of Rasulullah (sallallahu ‘alaihi wasallam) should therefore only follow him in every aspect of their lives, including eating and drinking.