Hello Brooklynodog, greenhouses and tunnels can get very, very hot. The purpose of damping down is to reduce temperature when it is too hot. And also hot and humid is much less damaging to plants than hot and dry.
So unless it is too hot for the plants you have in there, there is no need to do it. But it can be a lifesaver when it is to hot.
All depends what you are growing. Tomatoes don't like temps above 25.
Can't tell you anything about blight. Never had any experience of it.
Hope it all goes great for you.

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