So let's see: The insignia, according to 18 USC 701 is to be used by authorized and official people to invoke and convey the authority of the agency. When an FBI agent asks me to accompany him/her and flashes the badge, it invokes their authority to ask that. I am supposed to comply, or bear whatever consequences.
BUT: If no one is supposed to display the insignia, then how would I KNOW that the badge is a tool of invocation of authority?
The FBI wants everyone to become a mindreader, huh?
Godwin's reply is proper, measured, factual, and conveys all the seriousness the FBI's reques deserves. This is ashow of muscle, an act of intimidation, his response is simply "Don't f**k with me, I know the law as well as you! Go away..."
There is an old story about some yo-ho who received one of those pictures-tickets for $xx for driving through in a red light - the camera caught him. He did a dumb thing: Sent in a picture of said amount (not the actual money, or check.) The Police department (I think it was Los Angeles PD) send back a reply: A picture of handcuffs. Compliance achieved! :-)))
I think that humorous reply, PROVIDED IT IS TO THE POINT, is perfecly good and perfectly acceptable, certainly not illegal, and in most cases will get either a humorous (and to the point!) response, or a dry business response. There is no way any clerk, IRS, FBI, whoever, will risk treating a citizen (or anyone else for that matter) with less than correctness.