There have been some reports that ram is not being made available to the system after some few vm's are shutdown, that is not normal. I have not seen that myself. In some unique vm's like bare metal there may be some overhead too. Some gui's for VM's might use a bit of ram also.
In normal use, a vm that is turned off will as TobySGD points out, will not use ram just like any other unused program.
Last edited by jefro; 04-08-2013 at 05:06 PM.