One doesn't need a dual-stage regulator but it eliminates potential headaches like end of tank dump - which is very real. And for the price, I can't imagine not just getting dual-stage. The price difference is so minimal that it just makes sense. Kind of like spending a zillion bucks on a paintball system when you could get a 10lb system for a similar price.
For my last regulator purchase, I spent a whopping $102ish on the regulator, manifold, three needle valves, solenoid and shipping.
Another thing about getting a full-size regulator… you won't have to buy a whole new rig when you decide to move up from a 20oz bottle to a 10lb cylinder.