You must be thinking of watering citrus orchards right as it is freezing in order to encase them in ice. Why do you think you need to protect your rhododendrons?

Heat is released when water turns to ice. Spraying water in freezing temps minimizes the temperature drop and can prevent freezing if not too cold.

Where are you? Most Azaleas can handle a light frost unless you are growing azaleas from the florist trade, and even some of those can handle the upper 20s.