From this you can then determine which tiller was shooting tightest. You continue until you make no adjustments to nocking point. IMHO tiller is a static relationship between the limbs and the archer, I see no reason why grouping should be tighter at one place or another. This can lead to problems of finger sliding into the nock and pushing the arrow on top of the button. IMHO tiller is a static relationship between the limbs and the archer, I see no reason why grouping should be tighter at one place or another.
Bad tiller means bad tuning and low bow efficiency.
Tiller is there to make sure the bow limbs push at the same time and stop pretty much at the same time. I initially set up tiller to give a bow that moves straight forward on shot, with minimal post-shot limb vibration. After that, the rest of the tuning pretty much takes over. But you're still free to tiller tune. In fact, some good compound archers I know recommend it as 'vertical micro-tuning'.