Trust can be a hard thing to build.
The longer you are with a company the more people tend to trust... there will always be the concerns of "can you confirm this?" or "are you sure this is accurate"... and sometimes there really is a mistake that needs to be dealt with.
However, I find that trying to be transparent in what you are doing, and admitting to mistakes when they do happen goes a long way to building trust. I also always make sure I make myself available to walk anyone through the report, how it was built, the logic of the segments, and even show the an end to end run through the site showing them what information is being captured to help put their minds at ease.
Basically, don't brush people off... take the time and effort to walk through the concerns, the more you do that (and the more you show why the numbers are what they are), the more they will trust you.