I will always use my trailing foot as the "spring" foot. I will do this for pretty much any situation....I don't personally like the method Flex describes (although it works fine for him) because I feel it is slower and also has a lot higher disaster factor as you are adding in movement to a shot that doesn't need it....this could cause a point down...or a mike depending on the situation.
Also, should you have a jam or trigger freeze as you are falling out of the box, it takes a heck of a lot more to get back into position, fire the shot, and then move again rather than to just move when you are done shooting.
I don't necessarily think either way is always faster (my way is faster for me because I practice it a LOT) but I do believe my way is safer and more consistent throughout a match.