I dont think keeping it in a plastic bag is good, the might release some harmful stuff, that could kill it, I dont know if this is completely true, but I know some plastic bags, can change the structure of other materials, and if just a little water will enter the bag it can come out easy and will harm your computer.
I think the best thing to do is to have it in some kind of material who can absorb any liquid coming from outside or inside
But then again, like Toni say, everything die before you know of it
But don't worry. I got over 30 years old Pong machines being stored in a cellar (before I bought them a few years ago) They still work perfectly. You diskdrev and stuff like that would die first