ExternalInterface.call works, but javascript can't access actionscript callbacks?

ExternalInterface.call works, but javascript can't access actionscript
callbacks?

In a webpage, javascript & as3 are setup like this:
Javascript loads the swf in the page.
swf calls ExternalInterface.call("javascriptFunctionName", "");
Javascript's javascriptFunctionName() uses actionscript functions set up
by ExternalInterface.addCallback
Currently, javascriptFunctionName() starts by checking that it has access
to the actionscript's callback functions.
Is this check necessary? Or does the fact that the actionscript managed to
call the javascript function indicate that access is granted?