First thing I did. So far so good, but hard to say if it actually helps, just something I automatically do with a new device. I'm consistently getting right about 10 hours on a full charge with my 6 Plus, under moderate mixed use — some games and camera use that eat battery, but also some low-CPU stuff like iBooks and podcast playback. That's 10 hours of actual screen on usage (~50% screen brightness), not standby time.
No because Apple actually only lets you charge to around 96% or so and then trickle charges to 99% and then drains until 96% again or something like that. If Apple just let the battery keep accepting a charge then yes.. But basically anything rechargeable nowadays will have safety nets like such in place.