This is not something you should be worried about. The engine will take care of any memory management for you, and aside from a few edge cases (Like the intro cutscene to WiH: Tenebra), it will do an acceptable job of managing assets.
(To answer your question more specifically, FSO uses a strategy whereby assets will remain in memory for as long as possible, on the assumption that any given mission will share a number of assets with the ones preceding and succeeding it. When memory gets tight, the least used assets are unloaded; this process can not be manipulated by users or modders)