I like it anytime folks are advocating extra caution when dealing with house (mains for you Brits) wiring. My old man has been a journeyman for 40+ years, my brother is licensed and following in the trade. I’m not a tradesman but I do my own wiring and electrical construction, and to this day, I can tell you none of us take chances when it comes to electricity.
Understanding electricity, how circuits work, and houses are typically wired is fine, until you get into a situation that you are not ready for, like as has already been described, a shared neutral. Or double tapped breakers. To this day, I won’t work on multiple areas of my own home or ever work at a side job without flipping the main at the panel. It is too dangerous not to, and in most households simply requires the resettng of alarm clocks. Computers and most sophisticated modern electronics maintain time and date settings without the need for constant AC power.
If there is any question, any question at all, about whether a wire is live or not, Then for the love of God- TURN THE JUICE OFF AT THE PANEL before working. Yes, for installing a cieling fan or replacing a light fixture for which only one set of wires are involved it is uncalled for, you are BETTER SAFE THAN SORRY.