Wouldn't that depend on old school or new school? Just like for groundies?
Martina is definetely old school, where you move your feet for every volley.
Modern volleyers would like to move their feet for every volley, but sometimes the ball comes just too plain quick, and it's all hand to block off a winner.
Then you add volleying off hard backspin balls, and hard topspin balls, where a longer sweetspot/strikepoint is important, so bring back the feet.