I disagree. I always trust people unless (or until) they've proven themselves untrustworthy. This may sound naive, but it's what works for my style of doing business, which hinges on mutual trust and almost over-the-top openness.

Here's what "guilty until proven innocent" misunderstands: without trust, the relationship is doomed regardless of whether the mistrust is justified or not. If I don't trust someone, I don't do business with them, period. And if I subscribed to your "simple rule," there would be practically nobody I could do business with.