Very seriously, there's 25 things to worry about every day, the battery on your Apple Watch or iPhone isn't and shouldn't be one of them.
As all previous posters have aptly said, charge it when you need to. Ideally get into a charging rhythm. Apple, and others, put optimized charging features in their devices. Part of this is seeing your schedule and waiting to fill the last 10 or 15% of battery just before you typically start using the device. This significantly reduces the amount of time your device's battery sits in that less than optimal full/just about full charge area.
But! even if you charge it out of rhythm sometimes, don't worry about it. The worry is far more costly than the minor wear you'll put on your battery over the years.