Pretty much the title...
Currently, I replace the batteries in all my HA devices annually, whether they need it or not. (A couple of my locks get a fresh set in the interim, but even those get replaced again during the annual cycle.) It's not a big deal, just one weekend afternoon of time and a near-meaningless dollar amount.
Given that -currently-, I don't have anything setup that would be fatal if a battery device dropped off for a day or two, I've been considering moving to the 'as needed' replacement cycle. (ETA: For clarity, the non-locks should all go longer than the annual cycle, likely between 18-24 months.)
I do understand that lithium batteries have a pretty steep tail on the discharge curve and a reported 50 (or whatever)% is really 10 (or whatever)%. I also have notifications setup for low batteries and get notifications if a device hasn't reported in regularly. (I think I have it at 12 hours for battery devices.)
I'm guessing that my (assumingly) relatively low device count plays a factor, as well. I have 17 devices that take various lithium batteries and 3 devices that take alkaline batteries. I also know there's no right or wrong answer. I was just curious how others approach this.