Rocking a duck stance is best for stability, spinning and definitely switch riding. Most pros out there rock 12-15 in the front and 9-15 in the back (I ride 12 & 9).
As long as your board is set up to be a true twin (the measurement from the tip to binding and tail to binding) is the same, a few degrees difference here and there won't really make a difference in feel. Your stance is all about what is comfortable for you with your set-up (board and bindings). Some guys love 15 and 15, while others couldn't imagine going that duck.
Try out a few different stances and see what you feel more comfortable with. I recommend staying in the 12-15 in the front and 9-15 in the back range.