IMO, Blizzard keeps these things relatively ambiguous, otherwise, let's be honest, the world first guilds would have the credit.

So for the most part, since most raiders geared up for ICC through ToC, all raiders who went into ICC were members of the Ashen Verdict and therefore the Ashen Verdict, and by extension, the leaders of the Ashen Verdict take ultimate accountability and credit for Arthas' death. At best, the raiders themselves are the anonymous "task force" that ends up being the support and the "cannon fodder." In any event [spoiler] anyone who makes it through the LK fight learns that they are the crème de la crème of the AV forces, and as such are tagged to be the greatest of the Lich King's subordinates - but then Blizzard set that up from Day 1 in Howling Fjord (if you're not a dope like me and actually get yourself "killed" by the LK during one of the quests - otherwise, I *think* it's the necklace quest in Zul'Drak that drops the first clue that the LK has his eyes on you).

So as far as a take-away, Tirion Fordring takes ultimate credit for the kill. You, the raider, are the Ashen Verdict "Special Forces" that did the dirty work.

From a quasi-role-play perspective, in essence players have to do a little bit of ret-conning. Instead of thinking you've killed the same bosses dozens of times, the perception is more along the lines of the cumulative time spent from starting trash to ending boss represents the initiative - so even though we may say we killed end boss A 37 times in the last 3 months, as far as a storytelling perspective, we spent 3 months hunting down and killing the end boss. At least, that's how I perceive it.

This, of course, is all rendered moot if Blizzard clearly states who killed something (ie: Varian Wrynn killed Onyxia).

In any event, there is a lot of willing suspension of disbelief once you get into raiding, since Blizzard isn't yet at a point where the bosses will remember you from the last time you ransacked their homes...and I'm not sure we want to go there. Even though it would be funny for a boss to say "what, you again?" You know that it wouldn't take long for the Goblins to start selling bosses "dungeon-owner's insurance, and I'm sure that's going to play rough with raiding economy. Insurance isn't cheap, you know.