When CJ say flat, he means real flat, not sorta flat. Especially so in the areas where you walk. The perimeter is more tolerant, but still, you want it flat.
Your flooring should have instructions as to how flat. Flat to 3/16" in 8 or 10 feet is common.
Manufacturers and their advertizing make it seem easy as pie to install, but the don't bring up the most important part of the installation. You can easily spend more time prepping the floor surface correctly than installing the floor.