Here is explicit acknowledgments that a bit of flaring is OK, but "excessive" flaring is not.
Here, the fist rotation is described as commencing at 75% extension of the striking arm, so if the elbows do point sideways, flaring will be minimal.
One reason boxing and some other styles (including karate) recommend turning the wrist at the end of a punch is because a fist (gloved or ungloved) that is twisting at the moment of impact is more likely to tear the skin and/or cause subcutaneous damage.
Flaring the elbows is often criticised for:
Reducing defensive integrity
From a purely boxing position - I would suggest it doesn't matter at all when the fist rotates as long as it's not affecting the efficacy of your punch (I've noticed in my students that if the fist rotates too soon, it can lead to a "knocking on the door" motion where the strike is with the tips of the middle knuckles rather than the flat section ...
After extensive research, I found some enlightening pieces of information thanks to GoogleFu.
The only written and half-decent source I could find is an article by Thomas Green from 2005.
There, we have this bit:
Traditionally, the lead leg (the left in the case of a right-handed boxer) was wrapped by a chain extending from ankle to knee. Known as