depends on what level you are riding at...if you want to land switch more its alot easier if your in the negatives, and it does help for balance on the rails, its usually personal preference, i have 21, -21.
i have 18 on the front, -9 in the back. 22"wide (yeah i have a wide stance for a girl)
pretty comfortable with it, and i can ride switch alright.
my advice: do what feels comfortable. if you do decide to duck your stance, don't duck it too much, because 1.that's bad style and 2. probably very uncomfortable and very bad for your legs. super wide stances are a silly trend, go for shoulder width stance.