Mount the 3 way switches horizontally. Then the issue of "up-side-down" goes away.
But then you can argue about whether both switches left or both switches right is off, or whether one left and one right is off!
I've found the thing which confuses some people is where you have
two 3-ways on a dual plate, e.g. for ceiling and counter lights in a kitchen which has two doorways. They sometimes can't grasp that depending upon the position of the switches at the other end of the room, there will be a time when you have to move one switch up to turn on a light and the other down.
