My Pixel 2XL is coming this week. I don't think you have to charge it up completely but someone who has it can tell you.
In regards to saving battery life
-swap the animated background wallpaper for a static one..
-Having a black or dark background will save more juice over any other background
-you can turn off the always display and music
-use low brightness settings
-trim/close any apps running in the background
-uninstall apps you never use
-put it in airplane mode occasionally
-turn off unnecessary hardware radios like NFC, GPS bluetooth, mode until you need it
-update apps
-when in doubt, you can put your phone on battery saver also