We don't have any minimum length limits on the lakes I guide on but for the lakes that do have a minimum length limit the regulation requires that the fish be measured with it's mouth closed and the tail lobes pressed together.
It's easier for me to measure fish (for bragging rights) with my thumb in their mouth to hold their bottom lip up against the end of my Golden Rule so I measure them with their mouth open but I always round down to the nearest half-inch.