My thing is not so much worrying about who gets last hit so much as not being able to see if you are ahead or behind the curve in killing him. We have had a few times where some of us gemmed the last few rounds to make sure we were able to kill him because for whatever reason we were light on participation that day. If you can't see how much you have left to go, you run the risk of not being able to kill him and reverting back to a smaller boss. You lose a lot of rep when that happens.