You have to assume the pilot is aware of the local terrain and is therefore not going to request a heading that will put them in a dangerous position.
Of course! I have no problem to assume that. I KNOW the pilots are the least interested in having an accident: that's taken for granted. I would be happy if ICAO came with a clarification on this issue stating something like
"when a pilot requests deviations from his/her route due to adverse weather, even if the airplane is below the minimum safe altitude or minimum vectoring altitude, the controller should approve them unless conflict with other traffic exists".
However, not all pilots are familiar with local terrain. If one of them requests some heading because of adverse weather ahead and I approve it (assuming that he knows what he is doing), the pilot might think that flying that heading is safe. That's the danger of the double assumption.