In fact, any menu button will always hide the keyboard when its active.

But mapping the toggle action to a menu button is certainly the right 
thing - and a smart one, too:

1) at first menu button press, the action is executed and shows the 
keyboard

2) at second menu button press, the press is handled by the virtual 
keyboard itself. As its a menu button, it'll hide the virtual 
keyboard. But as that's the same as the toggle keyboard action would 
do on a button other than left, right, up, down, menu, you won't be 
able to tell the difference.

Best regards, 
Felix Schwarz