ah, chomik, finally someone that thinks lang lang's show is too exaggerated. he does seem unnatural.
unnatural/natural aside, my teacher always said you should let your whole body play, but just your fingers. breathing, arm movements etc should be took into account when playing. im not sure if the music sounds better when one is throwing their head/body/arms around with the music when playing, but, i unconsciously do that sometimes. i do that when im merely playing in my own room, by myself. it's definately not for show. if it comes naturally for you when playing, and it's not affecting your playing in a bad way, then i guess it's not a bad thing, and can be called 'technique'.
if the movements are just for show, then just forget it. it doesnt look nice when a performer exaggerates body movements to add effect to the peice (most frequetly, it does look fake)