Does it reduce system overhead to reset the rowsource of list boxes on close?

I am building them on load, and thought it might make sense to reset them to null on close but that gives me an error:
"Run-time error '2467':

The expression you entered refers to an object that is closed or doesn't exits."

The function appears as follows:
Private Sub Form_Unload(Cancel As Integer)
Me.PermUserID.RowSource = ""
Me.PermissionList.RowSource = ""
Me.CurrentPerms.RowSource = ""
End Sub

I can just abandon this if it doesn't really buy me anything.

I'm not aware of any profit in clearing the sources of listboxes, and it will very slightly increase your network demand - your files are a tiny bit bigger with the presence of the form unload code, and it will take that little bit longer for the file to close.