I've always thought that no response was required, except for pushing the ident button. If you've heard the request, the controller will have his reply when he sees an ident. That's all I've done for about 25 years and it's never been questioned by a controller. And it keeps the airwaves quieter.
BTW. Do you know whay a squawk is called a squawk? Well, it's because during WW2, the first transponders were a system called IFF (Identification Friend or Foe) and like many pieces of classified equipment, it was given a code name. IFF's was parrot. So when you parrot was interrogated, it gave out a squawk!