Some poeple go with longest travel being X.
Some people believe X is the one that is 'cross ways' when you're standing at the console/end/controller of the machine.
Some say that Y is always the travelling axis on the gantry.
Some say X is an unknown number.
Some use a small crystal, hanging from a birch tree on a full moon night to determine the optimum placement and axis alignment for their machine...
ie: toss a coin.