I turned mine off. I don't know if it really helps save battery or not, I just found it annoying.
I think iOS 12 does some other things in the background that eat up battery. I have two iPhone 8's here at work. One is on iOS 11 the other iOS 12. Both have been reset back to factory. Just letting them sit on my desk with nothing installed on them, they're just connected to WiFi. The iPhone 8 with iOS 12 will drop from 100% battery down to 96% in a couple of hours, while the iPhone 8 running iOS 11 is still at 100% battery and will stay at 100% battery until the next day, and by that time the iPhone 8 with iOS 12 is at 73%. And again, nothing is running on these phone, no apps or anything.
It's just an observation I have noticed.