It really depends on the water quality in your area. For a closed heating system TDS (total dissolved solids) is not generally a problem unless its steam. For domestic hot water, The hardness will tend to precipitate out on either the heating element if electric or on the metal adjacent to the burner in a gas heater.
As a rule, domestic water heaters should be flushed every few months, but thats really never done.
So whats your real question???