I just leave it on, because if I do it manually and forget it's not on auto, then I pull out my phone to do something quickly, it's really annoying to have to adjust the brightness, especially when it's at like 10-15% and I am going from dim to bright environment. I don't know, maybe I just need to learn to have more patience.
However, on the T-Mobile app, when I do a device health check, it shws auto brightness as being the preferred way for battery life.
The green to the left means that's currently on the preferred option.