I think with a vaporiser that shoots out sublimate at speed it doesn’t do any harm to seal the entrance up. If I don’t enough escapes to blow in my face and I’d rather avoid that.But I agree that when using a pan it’s likely not necessary
Just done all mine a couple of times, circa 25 hives using GazVAp, but fitted extension tube for Nucs on at all times so it pushes the gasses deep into the hive, no need to seal up nice and quick