Grounding light switch to electrical box?
You can add a ground wire to the box from the panel that feeds it if you want to. 
If there is conduit it is using the conduit as the ground. 
With older homes you are not required to update the wiring in this case add a ground wire if the box is not connected to ground I almost never suggest lighting be updated to add grounds but regularly do suggest receptacles have GFCI protection added and or a ground wire added (the ability to add a separate ground is somewhat new in the past this was not code compliant) 
So it is up to you.