Whatever works best for you is the real rule, but this never means stay stuck in a rut and don’t experiment. Anyone that argues strenuously that there is only one ‘right’ way isn’t someone you probably shouldn’t take too much advice from anyway.
That said, what is easy to understand is that having the foot on the side you are casting forward is good for limiting/calibrating/blocking your rotation, and the opposite for giving you added ease of rotation. Both have their advantages and pitfalls, and both can be an expedient cure for certain issues that crop up. Standing neutral, with hips facing the target, another one.
Simon Gawesworth usually recommends an “open” stance when teaching the basics, even though we know he could do it standing one-footed on a pointy rock (and on either foot) just as well, and Travis Johnson a closed one. QED, mic drop, ‘Nuff said.
Ironically many of the most powerful casters like Travis seem to like the closed stance which superficially seems like the one that might limit your power more, but maybe for these guys (Travis used to be collegiate wrestler) blocking things in that way gives them an added confidence in the control when they “let the big dog eat”. But just guessing there.
Honestly, like people have said above, for real fishing you are going to get forced into casting a lot of times in crazy positions so best to not get too hung up on it. Some rivers like the Deschutes may tend to make you burst into laughter in certain places if you start thinking about the “correct” stance.