tagging others mobs is rude... its that simple. its not about the exp, just the principal. just use good judgement in helping or not helping based on rate of decay on the mob vs the player. just keep in mind that if you seek to please everyone you will always end up pleasing no one. there will always be the guy that gives you &&&& for something and just take that opportunity to /ignore them or just blow them off. i tend to ignore someone coming up to 'help' me as not important when its not needed and appreciate it when it is.
however i do hate when people help with every mob you pull or come up and 'help' you and then follow that annoyance up with just the words 'invite me' or 'invite'. boy howdy would i like to punch those fools.
one such feature that i miss from Everquest though was /y or /yell. what it would do is if things get hairy you can hit /y and it will let people within something like 10 meters that you needed assistance and where you are at. if you want to go with 'well people will just spam me with it', i know that i get it in emote spam all the time.
Really good leaders can lead without anyone even realizing the fact. And the elite few that make their grip on power known, exert their power in such a way that no one really cares they are being manipulated.