My ex was a plumbing contractor - he always said "COLD WATER"
(well there was a "term of endearment" after that.
It was the grease thing. If you grind up grease with hot water it will go through the disposal, hit the pipes and as soon as it finds a cool spot it will solidify and eventually form a clog. If you use cold water it will flush through the pipes in small globs and not clog the pipes.
That was what he pounded into my head over 13 years.