What are the requirement for electrical grounding of aircraft in hangar storage?
In general any aircraft parked in hangar should be electrically grounded. The grounding should be done from basic aircraft structure to a low resistance ground.

Grounding of the aircraft is usually done to,


Protect aircraft and personnel against hazards from lightning discharge.
Provide current return paths
Protect personnel from shock hazard
Prevent accumulation of static charge


During de-fueling or refueling, the aircraft is grounded to the fuel truck.

In general, the aircraft are grounded to a point where the impedance is less than 10,000 ohms referenced to earth for static grounding. For power grounding, it should be less than 10 ohms to power systems neutral.

Aircraft are not grounded to the hangar structure as the external power supply, when connected, is grounded to that. 

The best reference, if you ask me, would MIL- HDBK-274(AS)- Electrical Grounding for Aircraft Safety.