Manners vary from region to region and country to country. One can be easily embarrassed by violating certain behavioral norms. Still, most "good manners" are based on the simple respect of others and not exhibiting too much familiarity to others. You might "Sup?" your "bro" but, certainly not a new client or someone you haven't met or are meeting for the first in a "service" position or not.
As long as I am shown a modicum of respect, I do not expect slavish behavior, I'm reasonably happy. Like some, unexpected good manners are to be commended. Social necessities or manners, smooth commerce, encourage reasonable social intercourse/behavior, and just make people easier to take. Simply my opinion and it works for me.
It's gratifying to see so many "old school" types in this bunch. Our moms and dads did well I think! And DIs, don't wanna leave out my old DIs.