Ideally the device won't let it discharge to zero, but will shut down before it gets that far. And you do have to let it discharge to shutdown, occasionally, so the electronics know how much capacity the battery has.
My runtime just "magically" increased by about 20% by doing that. I had been consistently at 75-80% by the end of the workday. It started dropping, heading down to 60% or less, in the same time, last week. Looking at the consumers: There wasn't anything excessive going on. So I let it die, let it stay off for a couple hours, charged it back up, all in one go, and now it's back to 75-80% at the end of the workday.