Are helicopters reinforced asymmetrically? Helicopters are under a constant torque due to the action-and-reaction forces caused by the rotation of the rotor(s). In the tail rotor and tandem rotors configuration, the airframe is experiencing tension on one side and compression on the other. A weak airframe would be twisted by the strong torque produced by the rotors.

Are opposite sides of a helicopter airframe reinforced differently do due with the difference in stress?
As far as i know, the helicopter airframes are not reinforced asymmetrically due to this reason. Note that the helicopter has to yaw in both ways- so there is no point in reinforcing only one side.

The helicopter airframes are not weak- they are designed to withstand the full envelope of expected loads. Also, it is easier to design and manufacture a symmetric helicopter structure. 

That said, I've personally seen at least one case where the torque has affected one side of the helicopter more than the other locally (especially in tail fin area), though the effects were not considered serious enough to warrant a design change.