Lower it in the training room. There's a good spot in the top right corner where you can look at the bots.
On 100 aim assist you'll see the reticle drag, even if you don't move the controller.
Lower it, gradually, until the auto correction fits your playstyle/character.
I feel high aim assist is ok for characters that you don't need headshots or flick aim with.
However, if you have good headshot accuracy from muscle memory, aim assist can mess up your shots by instead dragging to the body, or drifting to another target.
Lower AA settings are good for mccree and widow.