I think a turn off the airway at the start of an emergency descent is necessary. My reasoning is that the turn reduces the risk of galley carts floating about and it gets you away from the very accurately navigated traffic below you on the airway. Turning back to parallel track about 5 miles or so displaced also keeps your LSALTs etc valid.
BUT if the instructor is set in his ways, go along with him and pass - that's what we're all in the sim to do.