It's important to teach manners and etiquette to all children, not snobbishness. Like it or not, you do them a disservice if you deliberately choose not to because they may well find themselves moving in circles or situations where it is the norm or expected and if they don't know the "rules" then chances are they will feel uncomfortable, embarrassed or insecure.
Once you know manners and etiquette you can judge when it is ok not to use them, or of course choose not to use them if you disagree on principle. But knowledge is power.
Many restaurants these days don't serve or cater on such a way as to make standard etiquette easy or appropriate. Eg the pp who made the valid point about the haphazard delivery of dishes at a work Christmas party. In such instances, the politest thing to do is to wait until those in your immediate vicinity have been served also so that those you'll be sharing the meal with eat together. Of course, if one of these people is left waiting for their food for a long time, for example, then they politely encourage the rest of you to start.
To start eating at the same time is about sharing a meal with others and ensure that no one is left twiddling their thumbs between courses while waiting for the rest of the table to catch up. The etiquette is there to ensure the smoothest and most enjoyable dining experience for for everyone. If the service let's the diners down, this isn't the fault of etiquette but demonstrates poor service.
My family were always very strict on manners but we were never made to feel bad or ashamed, they were taught naturally and gently from the cradle. My friends and colleagues come from a variety of backgrounds, and I adopt the etiquette that ensures the most mutually enjoyable experience with them.
So of course I'd never point out ignorance of etiquette on their part (even though the friend who always takes a large chunk of the nose of brie drives me nuts lol). And in informal social gatherings I don't formally greet people I meet but will adopt the manners that put others most at their ease - and myself! It can be just as embarrassing to feel overly mannered (read: pretentious) as it is to feel lacking in social graces.
It isn't pretentious or stuck up to be armed with how to behave in different social or business circles, but it definitely is (and terribly rude) to insist on such manners in situations where it's uncommon or unnecessary.