I'm making a call:
myResult = MakeMyCall(inputParams, out messages);
You have to declare a variable which you will then ignore. This is most commonly the case with the TryParse (or TryWhatever) pattern, when it is used to test the validity of user input (e.g. can it be parsed as a number?) without caring about the actual parsed value.
You used the word "dispose" in the question, which I suspect was just unfortunate - but if the out parameter is of a type which implements IDisposable, you should certainly call Dispose unless the method documentation explicitly states that receiving the value doesn't confer ownership. I can't remember ever seeing a method with a disposable
out parameter though, so I'm hoping this was just an unlucky choice of words.