When I play SC5 with other people it's mostly to goof off. Winning and losing are pretty low on my list of priorities. Pretty much everything I do revolves around goofing off. That includes overkill. So whether or not I do really depends upon who I am using. For example, if I am using Mousy, I usually just bounce up and down saying, "ME WIN!! ME WIN!!" If I am using God then I will sometimes overkill them with very outlandish sword moves saying stuff like, "you will KNOW my name is the LORD when I lay my vengeance upon thee."
Even so, overkill is only part of the joke and I know some people dislike being over killed at all, so I respect those people's wishes while other friends are in on the joke and often do the impersonations themselves. LOL