As others have mentioned switching polarity for keyboard is akward. I tend to move my finger from my D (Right Key) to the F (Polarity Key) making it difficult to move right and change polarity in the last level. like some others have said having this option on the mouse whould make a lot of sence as the mouse controls the magnet, But that might make it hard for laptop users. I think a nice middle ground would be to have the default key binding be the LEFT SHIFT key as the pinky does not have a critical role in moving.
Summary: Changing polarity with mouse or shift key are easier to control as the default settings.