setting Half Way Rls With IS to ON causes the image stabiliser to be activated whilst the shutter release button is half-pressed
this is EXTREMELY useful when attempting to do manual focus with a magnified view of the image with the camera hand held
should you set it to ON?
I personally leave this set to ON all the time
HOWEVER, if you do not do magnified view manual focus then you could leave it OFF to save battery life if you are in the habit of holding the shutter release half-down to lock exposure or AF for long durations