Charge when you want. Let it run down as much as you like.
You will never actually get the cells to 0% as the watch realy has around 10% in the cells when the display says 0%. Its just how modern devices work.
You will hear a lot about "Dont EVER let it get to zero! Dont charge to 100%" but that info is a half truth. True, you should never let a Li-ion battery get to 0 or 100 but YOU cant ever do that. The device will stop charging before it gets to the top and never let you run it all the way down. Now, if you somehow could bypass the built in battery management then sure, you can damage the cells. But you cant.