Onions fail at the neck. Whatever moisture the bulb gives off after it is packed collects there first, and once the neck stays wet the rot starts in the middle of the pallet where nobody can see it. That is why onion has been packed in mesh longer than almost anything else in the produce aisle, and why a pack that looks better but breathes worse is a false economy.
The real decision is not whether to ventilate — it is how much print area you are willing to trade for air, and whether the line is limited by throughput or by what the retailer demands on the shelf.