personally, I have never understood an unequal duck stance (ie 18/-9, 12/-17). To me that is straight up uncomfortable.
I ride 15/15 wide stance, and couldnt be happier. easy to ride switch and do 180's, feels balanced and stable.
I think it might have to do with peoples hip rotation. If you have a wide stance but can't keep your knees over your toes its going to spell trouble.
You should not be forcing your knee over your toe with twisting your feet, it should come from the rotation of the hip socket.
I am naturally more flexible on one side that the other (also knew this from years of ballet and leaning how to turnout properly)
I ride +15 and -12 on the back (think I got that the right way round, at work, cant check). I have fiddled with it and tried other stance, I eventually set it to where I would bend without forcing my knees over my toes and feel comfortable riding switch.
It might be like this for other riders too?