On my older bikes (Honda CB350 and my air-cooled BMWs), the gas petcocks were "manual only." In other words, they had off/on/reserve positions, whereas newer bikes have on/reserve/prime positions and the fuel flow is controlled via engine vaccuum.
While it is true that the float needle-and-seat SHOULD keep gas from running when parked, sometimes a wee little bit of "crud" can get stuck between the end of the float needle and its seat. As a precaution, I made turning the petcock to off as part of my shutdown procedures, "just in case."
Just "food for thought.".