fin/wing tanks freezing

Had a Nimbus 3 in the shop some years ago with massive structural wing
damage due to freezing tanks. Took the upper wing shells clean off the
spars and/or cracked the shell right through just in front of the spar (the
width of the tanks), and the resulting 'new' aerofoil made flying it very
interesting according to the pilot. Interesting repair as well.

Damage can be massive, control problems and weight/balance problems would
be an issue too with freezing water. Most flight manuals specifically
state: water to be dumped before reaching freezing level. Even when not
using common sense these factors should be enough to make up your mind, I
would say.