With the disparity you are concerned about.... if I didn't move a decimal the wrong way, over the course of 100 miles (160.9 kilometres) you will be off by 29.4 feet (8.97 metres). Do you need more accuracy**********???
Measuring the wheel circumference off the bike will be a disparity from doing a roll out because the tire isn't that circumference when weight is it giving it a smaller diameter. Is the difference material? Probably not.
The only thing I might say about auto calibration is that it might be that one day the auto calibrate gets flaky. Maybe for GPS satellite interference with environmental factors. And it might be that it goes flaky when you are doing the best most important ride of your life. Murphy certainly must have a law about that!
I had it happen exactly once. A long long time ago. I'm pretty sure they have built in some additional code to look for such as I've not seen anyone complain about it like I was seeing ten years ago.