I haven't used it for a while, but from my testing of older versions it never leaked.
But browsers can change overnight.. so instead of "finding this out the hard way" or just taking my word for it, you guys should use a site like whatismyreferrer.com and test it out with every browser you can get your hands on. If these were my CPA accounts that were being put on the line, I would want to see for my own eyes that it doesn't leak.
Well my CPA company found were my traffic was coming from so at times I think it dose leak.