Are helicopters reinforced asymmetrically?
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.