I'd pose the question in Mil Pilots or Rotorheads, for a military response - BEagle tends to be one of the few who ventures here.
If the choppers you've seen are military, I'm surprised there as high as 500' - tend more to be 250ish. From what I have seen and know, they are all professional aviatiors who understand their environment and the risks they pose to other airspace users.
And although it depends on your definition of "close", I don't recall a military helicopter being involved in an airmiss with a civil aircraft - although a Tucano did clip the back end of a police helicopter a few years ago. I'd worry more about the jets.