[Jim Malmstrom]"This has been a nagging a question I've had: Is the RAM that you assign for AE's use not available for your OS or other app's even when you haven't started AE, when AE is in an "unopened" state?"
No. AE should only reserve RAM for itself when it is open. When you quit AE it should release the RAM for usage by other applications and your OS.
Sometimes if AE crashes, or you have to force quit, the additional rendering processes used for "Render Multiple Frames Simultaneously" may remain open. You can check this in Applications > Utilities > Activity Monitor on OS X, or in the Processes section of the Task Manager on Windows. If instances of "aeselflink" are still running you may want to kill them.