It is generally recommended to leave the microwave's vent open during use. Microwaves generate heat, and during the cooking process, steam and moisture can be released from the food being heated. The vent allows the steam and moisture to escape, preventing excessive condensation and potential damage to the microwave.
Leaving the vent open helps to maintain a balanced moisture level inside the microwave oven and prevents the buildup of steam, which could potentially affect the performance of the microwave or lead to issues such as warping or damage to the interior components.
However, it's important to note that the specific instructions for venting may vary depending on the microwave model. It's best to consult the user manual or guidelines provided by the manufacturer to ensure proper usage and ventilation for your specific microwave model.