Manners are in decline in general.
Do I teach my kids manners like when I was growing up and risk having them ridiculed because they will sound 'posh', or admit defeat and join the rest of society?
Grammar and spelling are also in decline - people throw their arms up if you correct either of these and deem you fussy or retentive. I think they are indicative of an individual and society as a whole - you care about communicating effectively so you b****y well make sure you can do it right.