You don't have to use the boards at all, they are only helpful in stopping leeway, lifting a hull, and pointing higher when going to windward. With the boards up, or left out, the nacra should only draw a couple of inches. Sailing in shallow water can be a blast, but if your rudders kick up completely you can lose control and wipe out. hitting bottom or obstructions at speed if you misjudge depth on sandbars can be both painful & expensive structurally. Being tossed off at speed in shallow water can also hurt. There's nothing like gravel rash to bring the point home. Having rudders partially or fully in the water at all times helps ensure you remain in control. I sailed at speed in a foot of water with the rudders only just under the surface, but it was tiring because of the huge effort needed to steer.
grinding away your rudders on the bottom is a no-no unless you are into rebuilding gelcoat and glass layers.

Enjoy!