That's a good question; unfortionately, I read about a case a while back where, essentially, the refusal to allow an unwarranted search was the justification (probable cause) for the search.
Sadly, I did not think to bookmark that case.
Given the state of our 'justice' system though, I would not be surprised if that were to become precedent and "well established law."
One of these days, I hope at least, people will get fed up with such abuse and hang those evil men who perpetrate and perpetuate such a system.
You ALWAYS refuse a consent to search. Reason being if they had probable cause they would not have to ask your consent they would just tell you they are doing it.