In the book What If?, the author usually writes the units out as kilometer, atmosphere, megawatt instead of km, atm, MW. However, he does use the symbols in the drawings:
In one line, both mm and millimeter are even used together:
The core of a lightning bolt is a few centimeters in diameter. A bullet fired from an AK-47 is about 26 mm long and moves at about 700 millimeters every millisecond.
This makes me confused because I can't find a reason to not use the SI symbols. I guess the reason is similar to writing numbers in words when doing so doesn't look silly for a better reading-flow-feeling. But in the above example, I can't explain why the first number should use mm while the second one should use millimeter.
Is there a general rule for writing out the unit instead of its SI symbol?