Re: Decline permission
""We searched because they said we could" is far too often sufficient probable cause."
Technically, that's not probably cause, that's them simply asking you for permission to search and you giving it. If they have your permission, they need no justification for the search whatsoever. At the heart of it, that's why everybody should explicitly state that they are not giving consent. As you say, that may (or may not -- but at least there's a chance) help later, in court, when the police have to justify the legality of the search.