When a recipe calls to reduce oven temperature mid-cooking, should you leave the door ajar until the oven reaches said temperature?
No, don't leave your door ajar. The recipes have already factored in the time an oven needs to go lower. Also, as a very loose rule of thumb, many dishes tend to do better without very wild temperature swings during baking, and some of them (e.g. steam leavenend cakes) react really badly to them.