An Apple official article for iPhone says:
When you use the Apple USB Power Adapter to charge iPhone, make sure the USB cable is fully inserted into the power adapter before you plug the adapter into a power outlet.
(from iPhone User Guide at https://help.apple.com/)
This seems to hint that plugging the cable and iPhone before plugging both to the charger has some potential risk? How so?
Best Answer
Looks like basic guidelines when handling electric devices: The moment with the highest risk of anything going wrong is the moment you plug something into the power outlet (or remove it from there). So it makes a lot of sense of not having your expensive device (the iPhone) already attached to the other end.