edit: Looks like it doesn't care. Interesting fact, if you bind an "keydown" event to something, it will fire when you hit caps lock to turn it on, but not to turn it off.
The correct shift key is the one on the opposite hand.
EDIT: I suspect most people use left shift because that allows them to type w/o removing their hand from the mouse (most people use the mouse w/ their right hand).
Example: K = left shift, R = right shift.
PS I feel compelled to go on record and say I'm very disappointed the 'For Science' badge is an image.