The "rules" often cited aren't because there will be a problem in most cases if you don't follow them. Instead, its because they will be problem in a very small (much <1%) of cases if you don't follow them, and the potential cost is very high.

(I'm not -- at least, not exclusively, or in general perception -- white, have lived in parts of CA with lots of diversity, and have often failed to follow the "rules" without problem -- and don't think any of that is unexpected even with the "rules" being valid.)