Morale Drops and Gains

According to what I understand of morale, if I have an army standing on an enemy province then that province's morale will slowly drop. But likewise, if I have an army not standing in one of my own provinces then that army's morale will slowly drop. How do they relate to eachother?

Is it worthwhile standing my army in an enemy province to drop their morale, or would I take more of a morale drop then them? And is it dependent on how many units in that army; therefore it would be worthwhile to just place one unit in an enemies province.

Thanks