Ever.
I once was involved in a Federal case and an industry-wide criminal investigation.
Had a former DOJ assistant AG, since gone into private practice, tell me that exact thing. He said: "you believe you're a good citizen, you committed no crimes, you have nothing to hide, and your natural habit is to help law enforcement. NO. Don't do it. You have no idea what they are up to. Tell them 'get a warrant.'"