I'm positive -2 dmg form an equipped weapon is always-on (at least for melee physical dmg when equipping this bow i have)
MY ranger wore final armor and tested this against Charr which have the Wild Blow skill (automatically does a critical hit) -- this method of testing was lifted from SonofRah who is an expert is damage calculation (although he never did answer my question on whether +armor from shields/foci/staffs were positional

)
Facing one Charr I consistently received -9 dmg when he used Wild Blow.
When I equipped the -2 dmg while in stance bow I got the same (no stance yet)... and when I activated stance I received only -7
These results never varied despite changing my orientation relative to the Charr.
However I did not test this with different altitudes... both of us were always on the same level
Extrapolation: shields should behave in the same way.... although SonofRah disagrees (I've read his article stating that shields do not cover your back)
I might test this when I have a warrior