Stems should be made tight enough that it requires a fair amount of force to twist it sideways if you stand holding the front wheel between your knees.
Less than that is mildly dangerous 'cause it might slip.
More than that is unnecessary, and can lead to unnecessary damage in the event of a fall. In a fall, the stem should be able to move sideways when it hits the ground. This can reduce the risk of damage to the brifters and/or handlebar.
It is silly to use a torque wrench for this application.
Sheldon "Tight Enough, But Not Too Tight" Brown