do i charge my watch right out of box

My watches have always arrived 70-80% charged with the most current OS updates. I could probably have gotten away with pairing, configuring, and installing my software without plugging it in but I haven’t. I’ve always immediately put it on the charger before proceeding. If it needs an OS update you’ll definitely need to have it on the charger.

Yes, it would be required to be on charge during any Watch OS update.