Local symmetry is accurate, but it requires that the subtool in question be a symmetrical object in itself, which a lone piece of shoulder armor, or an object rotated off axis in the world space, typically isn’t. Two pieces of shoulder armour mirrored left to right in the same subtool ARE symmetrical though, so it would be better to mirror the object from left to right to work on both at the same time. Tool > Geometry> Modify Topology> Mirror and Weld. You can always split them again after the fact. You can restore symmetry to a symmetrical subtool that has been moved off center by using Mirror and Weld with L. Sym activated, but it uses its local center as the mirror plane, so you may have to adjust to width to get the results you want.
You can store a morph target or rotate on a layer to restore an object to its original position, though generally the symmetrical modelling phase is over once you start rotating objects off axis in their worldspace. To some extent when using a layer changes made to the base will reflect itself on a transposed version of the model on a layer, but distortion is likely to occur. Better to simply finish your symmetrical sculpting before posing the model.
Also, read up on Transpose Master:
http://docs.pixologic.com/user-guide/zbrush-plugins/transpose-master/
which may have applications for what you want to do.