And long before "web experts" a lot of bullshit was talked as well by people in camera clubs and pubs and bars. There is a whole subculture of mystification of film - actually, photography generally, not just film - and what one should and shouldn't do and doing it right and doing it wrong that really irritates me. This tendency has been around as long as photography has too, so I'm not just some sort of old fart moaning about how "the internet" has ruined everything*
You can do photography in as simple or a complex way as one likes to make it, but it won't make one's photographs any more interesting or compelling.
Kodachrome is a good example where many people did shoot it under by a stop or two, but there's nothing like it available today and the whole engineering and processing of Kodachrome was amazingly complex, so it's unsurprising that one had to tweak it a bit ... It is to some extent true of the few remaining colour transparency films that one can do quite a lot to their colour and tonal response simply by (mostly) rather underexposing them, but that's really a consequence of the nature of the reversal process and isn't shared by negative films.
Modern negative films are amazingly robust in terms of latitude available and miracles of chemical and process engineering. There's a school of thought (mostly of photochemists who have actually been involved in film design and production) that modern films are so good that much of the photographic lore that is still bandied about and passed on as written in stone simply doesn't apply to the films we can buy now.
It is in anyone's interest to do a simple bit of testing with a roll or two to establish a "personal EI" - it'll allow you to compensate for any or all of the variables I mentioned above plus a few more I haven't.
The whole business of testing for EIs really only matters when one is wet printing. As far as producing negatives for scanning goes, the scanning process allows for a multitude of exposure and processing sins to be forgiven in the digital darkroom.
But there is no reason for anyone to just decide that, based on nothing more than what someone else says, they "should" shoot at a different EI from box speed.
*I'm actually a middle-aged fart