As you may have guessed by now you should never "bash" anything on a bicycle. And I would suggest that the mechanic who suggested such a thing is a hack. And since it is so easy to just loosen and adjust anything on a bicycle it's always a good idea to loosen rather than just brute force anything back into alignment.

Some may think that since it shifted due to the fall that it is too loose. I'd suggest that being tight enough to not move during normal use but loose enough to shift like this during a crash is a good thing. By absorbing some of the energy of the fall by moveing you redirect some of the force away from things like levers which could easily break. Since it took a fall to shift your stem around I'd suggest that it was tightened just right.