It's all about liability. That is the correct procedure, but 99.9% of the time it is not necessary.
Like the last electrician who replaced a breaker in my panel without shutting the power off. He said "I'll be fine" and he was, but if he was telling someone else to do it he would definitely tell them to shut off the main breaker first.
I replaced my MCU1 several times without going through any high voltage shutdown, or disconnecting fireman loop or the battery, or pulling any fuse, and it's fine
Just power the car down first; you probably don't want to pull wires off of it while it is lit.