This completely depends on the size of the microwave and the space in your kitchen or wherever you decide to keep it. Many microwaves are designed with a built-in ventilation system that prevents your living space from smelling like the food you heated in your microwave. It also cools down the microwave quickly and prevents it from overheating. If you have a large oven that you use frequently, then it is a good idea to install its own separate vent.