After entering engineering full time, I had forgotten cm is much more common in casual use (at least in the US), vs mm in engineering.

This is speculation on my part, but I consider mm better because it allows greater resolution than cm without fractions and makes more sense when you consider cm is the only non-thousandth metric prefix we use for common units (mL,kg,km - dB would be an exception). cm on the other hand is closer in scale to inches and generally easier to estimate because of this.