Is walk-in really necessary?

I see most pros do walk in, meaning take one step to put right foot on line, then place left foot. However, I've also seen some good players don't do walk in, they just have right foot on the final position while sighting, and place left foot and get down. So, with the 2nd method, if I can drop my nose/head down straight, is it necessary to do walk in? Is there advantage/disadvantage of both methods?

Example of 2nd method, practice session of Zhao Xintong:


http://ift.tt/1POGsGk

Thanks!
- Thunder

Is walk-in really necessary?