When would be the proper time to engage these items? Is this only when its snowing or freezing at airports?

well if its the MD11, as soon as the ICE DETECTED alert comes up:-)

Company policies vary. A common policy is to use deice whenever there is cloud and temp is between +5C to -10C. Always better to prevent ice than to try to get rid of ice, as the first step.

Just to add, in harsh winter conditions it's best to keep them on up to 10000 feet or turn them on at 10000 feet if descending. This I believe is done (as said above) to prevent an unwanted situation.

Thats the Answers I needed thanks Everyone!