Unless you have a way of removing the air pocket underneath the lid, there will always be the potential to develop mold in your tincture. Mold won’t form within the alcohol mixture, but it will grow on any herbs that poke out above the alcohol. The best way to prevent this is to keep the herbs completely submerged at all times. If you have herbs that tend to float, I find it helps to put a layer of leafy herbs, like sage, on top of them, creating a kind of barrier that weighs the other herbs down.