Here's one link, there are certainly other threads if you're willing to search for them that say much the same thing at greater length :-)
http://forums.omnigroup.com/showthread.php?t=10671
It is rendered moot by a feature in the 1.6 sneaky peek release that allows one to set on a per action group or project basis whether ticking off the last item marks the group/project complete (and to set the default for newly created action groups/projects).