[pjb]

There is a preference to "mark complete when completing last item" so if you know you've got all the tasks listed then the parent item (a group or a project) with this property will check itself off when you check off the last child. When you're not sure if you've got everything listed, make the last item "review this task list" like whpalmer4 suggests, and then take action when it appears on your to do list, or don't have it's parent set to autocomplete and then the parent will remain on your to do list as a less explicit reminder. I use both methods.