First off kudos for pre-checking your incubator.
For me, at the moment I use Hatch-rite, which is pre mixed with water. So I don't break the seal of the bag and fill the tub until I have eggs under one of my girls.
However, I am planning on switching to perlite in the future. So I talked to a veteran breeder about this topic. Here is what Colin Weaver said about this:So I can't imagine anything bad happening to the vermiculite by putting it in the tubs now. I just wouldn't add the water until you have eggs to put in there. Also just to be safe I would seal the egg tub with Press & Seal while the dry vermiculite is waiting in your incubator (you can also use that same Press & Seal to seal the tub once the eggs are in there to help maintain humidity).I pre-mix the dry ingredients in one of the 110 quart Sterilite boxes from Target and then scoop it out as I need it. I don't add water until close to the day I will actually use it.