I have a fractal design arc mini case, it has a front 120mm fan, a rear 120mm fan, and a top 140mm fan. It has a vent on the side where you can put a 120 or 140 fan, but since I'm not putting a fan there, should I cover it up or leave it open? I don't know if it would be better for airflow to cover it, or just leave it open to ventilate.
Do you have a graphics card in place? If so, I suggest leaving it open, if not, go ahead a close it up if your goal is to make it quieter. For airflow itself (sound not being an issue) I would leave it uncovered.