Yes I understand that.
Local Variables = More cpu time when the function is run due to allocating memory, 0'ing it and then releasing it after the function is completed.
Global Variables = More memory permanently taken up when it isn't needed.
I was weighing up the two options and thought that locals would be preferable since the plugin would probably be called once an hour or most likely less.
But don't worry since it isn't possible since query_client_cvar always returns the cvar value to a separate function.
Thanks for the help though.