Should I turn the iPhone 12 out of the box?
A new iPhone comes with around 50% charge on its battery. You can start using it out of the box and there's no need to charge it fully before starting to use it.
While it may not currently be applicable on an iPhone 12, if there's a newer version of iOS available for your model of iPhone, you may need to plug it into power outlet if you wish to install and update to the latest version of iOS. And that too would be required only after you have finished setting up the device.
But generally speaking, an iPhone battery is charged at factory to enabled it to be used out of the box while also ensuring that the battery chemistry remains intact while in shipment.