I had an conversation regarding FileVault with the geniuses recently too.
> Oh we see [your MacBook] has a password, would you be able to write it down here?
Haha, nope!
> Any, uh, reason not to?
Nope.
From a conversation later on, apparently not many people opt not to give up their keys. I'm not sure why they pushed me to give it up either, the geniuses know full well that they can just boot their diagnostics disk without the password anyway.
Bear in mind that the default setup for Apple's FileVault also sends a copy of the encryption key to Apple too (associated with your AppleID), where presumably there is access granted to the US government also (willing or unwilling).
