With as much as yours would siphon out, I'd strongly recommend the check valve, because just a little bit of algae or a small rock, or lord knows what else, and your whole tank drains.
Keith, your woodwork is furniture grade and looks slick! I was wondering if you put doors on the ends of the canopy to allow you to access the tank for feeding and coral placement, etc.? The scape looks great too. If I re-did my tank it would be similar with 2 coves like that.