What should I ask before washing?
Ask whether wash-cycle data exists and how resistance changed after washing.
Does washing always damage shielding fabric?
Not always, but abrasion and chemicals can change the conductive surface.
What should be validated?
Appearance, dimensional stability, surface resistance and shielding performance after the required wash cycles.
Project note: Use supplier data as a starting point. Validate the finished assembly at the relevant frequencies and environmental conditions.