I just use their default setting because my sites' visitors access my content through mobile devices.
My click-through rate had actually improved when I used their default settings using Auto ads.
I know a lot of people think that Auto ads suck, but you have to understand how Auto ads work in Google's favor.
If Auto ads show advertising on your content in places that it knows won't get clicked, where is the money in that for Google?
It's in Google's self-interest to get Auto ads to work at an optimal level because if they don't, they're leaving money on the table.
And so far, this has worked well for me.
Most of my sites are informative, and I don't build backlinks.
All my links are natural.
So depending on how you set up your site and the niches you focus on, your experience may be different.