Should I charge my phone battery to 100%?

No, or at least not every time you charge it. Some people recommend that you do a full zero to 100% battery recharge (a "charge cycle") once a month - as this re-calibrates the battery, which is a bit like restarting your computer.

But others disregard this as a myth for current lithium-ion batteries in phones.

To keep your long-term battery life in good health, frequent, small charges are better than full recharging.

With iOS 13 and later, Optimised Battery Charging (Settings>Battery> Battery Health) is designed to reduce the wear on your battery and improve its lifespan by reducing the time your iPhone spends fully charged. 

When the feature is enabled, your iPhone should delay charging past 80% in certain situations, depending on Location Services that tell the phone when it is at home or work (when you are less likely to need a full charge) compared to when you are travelling.

The deeper you discharge a lithium battery, the more stress is inflicted on the battery. So, topping up frequently extends battery life.

Credit: Techadvisor.com