Most manufactures recommend dumping once the temperature lowers to 4
degrees C. As noted by Ken, we have strategies for using water in
sub-freezing conditions, but there is some risk associated. These
include accumulation of ice on the tail due to leaks or freeezing shut
of the dump valves.

Using the 4C mark guarantees avoiding both problems. But as in all
things, good judgement and forethought can expand the envelope a
little. One simply has to understand and effectively manage the
increased risk.