In the Sierra, we fly every day at high altitudes with water ballast on board. Literally 10s of thousands of flights. 18,000 feet is common, 24,000 not unusual, OAT of around 20 F is normal. I have never heard of anyone that filled his tanks with antifreeze. Routinely landing with full tanks would be a far greater hazard. I suppose if you were contemplating a dawn-to-dusk record winter wave flight, it might be worth considering.

But perhaps we are all guilty of folly.