In my experiance, thinking about your pick hand is going to get you in trouble. Once you get the basics of regular, alternate, and sweep picking, stop thinking about it in depth and do whatever you need to do to get around the strings. "Master of Puppets" is played with all down strokes, even though its so fast. In my opinion this is an inefficient way to go about it, but it sounds and feels right to Kirk, so thats what he does. There are very few hard, solid rules on how to pick properly. No two people do it the same.