After 40 years, I'm coming around to questioning some of the things I was taught, learnt myself and even taught to others. Over the past five years or so I have listened to music more frequently on headphones than ever before. Headphone don't do stereo like speakers do. I think most people can work this out, but lots of my jobs, and even the hobby side are recordings of real musicians in a space. For me it might be jazz bands, small orchestras, choirs, brass bands - that kind of thing. Forever, I have banged on about the differences between X/Y, A/B and then later Blumlein and Decca Trees - with or without outriggers. I've come to the conclusion that headphones do make it possible to create the 'hole in the middle' people talk about all the time, but most times on location I do not have speakers, in the rehearsals I have sealed headphones (DT100's which many hate). When I am back in the studio, very rarely have I noticed any issues.
When I'm doing other kinds of music, I actually get better results with headphones - more clarity, and more separation. With practice, I can pan and adjust reverb depths pretty well now, so the mixes are fine on speakers too.
It's taken me a long time to realise all the doom and gloom stuff I even promoted years ago has nowhere near the impact I thought it did. Poor mixes, poor EQ and putting things in the wrong place are far more destructive than the speakers vs headphones thing. Perhaps I just wasn't as familiar with headphone mixing? Some of my stuff has been done on headphones and I cannot remember which is which - and I certainly cannot tell from listening. We just get told this, believe it - perhaps even using a bad mix as evidence (when it was just a bad mix) . I've stopped using this 'rule' - it doesn't seem to apply any longer?