Why unload swf's?

I’m using a setup where I load swf’s into my main movie, using the MovieClipLoader. Different swf’s is loaded into the same position. This means that only one swf is showing at a time.

Should I unload each swf when a new one is going to be displayed in it’s position?
What happens if I just load the new one? …will the old one be taking up memory?

Is it not good to keep the swf’s loaded, in case I want to return to them? So that I don’t have to load them each time I want to view them.

Thank you.