I'm not being facetious and I honestly don't know if I could prove it. It's simply a value we hold dear and we're up front about it with our clients and our team. We refuse to be dishonest, because it's bad for business.
For context: we're a small agency in a small city. If we either lied to our clients or produced dishonest advertising, we believe we'd be out of business in short order.
I'll give you an example. Many businesses suspect their agencies pad the hours on their billings. So instead we mostly do project-based quotes: as long as the scope of the project doesn't change, we don't change our quote, even when we lose money on it (and it happens). I can't prove that, though, so you'll just have to decide if I'm being honest about it I guess.