Depends. It doesn't have to be a rule but at a lot of places it kind of implicitly is.
I worked with two guys who became best friends by working together for several years just because, well, they saw each other more than their own families, y'know?
Really unlikely friends considering our increasingly tribal social bullshit, actually. One was the reddest of rednecks and the other was Hispanic. But last I checked, they still work together. And work on cars together after work.
There's a difference between canon and not-stupid.