Yes, the "Inform OmniFocus" checkbox does need to be checked (at the time the notification was posted, I think) to get the handy link back to the action/project. I don't remember checking that box, but it's been a while... And no, it won't automatically check the item off for you, but at least in my case, I'm often going to reschedule the item rather than complete it if the notification comes up at an inconvenient time.
I don't know of any other nifty Growl tricks, but finding that one sure made my day! If you get swamped by the notifications, you can hold down the option key while clicking the close button on one to get rid of them all. Unfortunately, you can't easily put them back up! Also, if you have to quit OmniFocus for some reason, when you start it up again, the notifications seem to lose their links.
Working in this fashion does tend to cause a certain amount of window proliferation, and I'm getting some use out of the "Close Excess Windows"
script I cobbled together in response to a forum post the other day!