No you don't need to do this incrementally
Put 1015 bios on usb stick, shouldn't matter if the usb stick has other files/directories.
Plug the stick in to your pc and boot into bios.
In advanced mode in your bios go to Tools -> Asus Ez Flash 2 utility
Browse your usb stick and select the bios to flash.
And yes if you bought that mobo recently with a IB cpu then myself I say update the bios as it's meant to offer better compatability with IB cpu going only from what I read in the Asus forum.
Regardless of what I say above you should really read the manual that came with your mobo..
I read the pdf manul for your mobo which I did from asus site and it's pretty clear as to how to navigate and update your bios from what I'm looking at.