It's all bullshit. I've got bikes with 165, 170, 172.5 and 175. It makes no difference to me
Good for you, count yourself among the lucky group of people with a wide tolerance for crank length*, not everyone is so lucky.
Changing crank length (going shorter, I'm 1.74m tall with short legs) for me sorted out** a persistent knee issue that would have me uncomfortable after 30miles, and in agony after 60, and struggling to walk the next day after 100 to someone who can happily ride 200+miles in comfort.
It may be only 5mm, but it's 10mm on the circle (or more if you change cranks by more than 5mm) and can affect hip and knee extension angles significantly and I can tell you the 'wrong' cranks are fitted within a few revs, and can probably tell you the length in a blind test with a fair degree of accuracy after a few minutes. I'm a lot less sensitive to it on fixed/SS or offroad though and can run longer on those bikes as I spend a lot more time stood up so the effect on hip and knee angles can be somewhat mitigated when standing.
If you're one of the lucky ones then that's great ignore crank length and crack on, if you're unlucky it pays to get it right.
* FWIW I'm the same with bar width, 2cm 'wrong' can give me pain, but not with TT or stem length, you can vary that 3-4cm and I'll likely be fine, other people are very TT/reach sensitive.
** was definitely the crank length that did it as it was a common issue across a number of bikes with differing setups in terms of reach and saddle-bar drop, and one by one I swapped them over to shorter cranks, riding one of the ones with the wrong cranks would bring the pain back until the cranks swapped and then pain gone.