I have registered and downloaded from LG what I need to unlock, but have not used it (maybe I will when I'm out of warranty or it'll be a plus when I sell the phone).Well losing warranty with the official method is for sure and even if someone would find a way to to relock it would not help since your IMEI is registered.
According to you I already lost warranty because my IMEI was registered? Don't think so.
You lose warranty when you unlock, not when your IMEI is registered.