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Hot water will remove more wax than colder water - but as long as its not boiling should be fine really.
What you need to be carefull is the temperature of the car itself - a car with warm or hot panels is a lot more difficult to clean than a car which is cool and in the shade especially in hard water areas. I tend to only clean my car when it is fairly overcast and cool now days - never do it in direct sunlight.