Such good responses so far.

I agree that:
1) Keeping key documents depends on what the customer wans and the function they would serve. Both regulated and unregulated industries can be protected by supporting data showing a company has performed due diligence. This is the case in qualiity, safety, financial and environmental.
2) Your plant is a member of a company - the parent organization should be making the decision regarding if, and what to keep. That's because it, not the closed plant, will be the entity paying the piper, so to speak, if missing records and documents are regretted later.