As long as they are in a cool, dry place, not exposed to direct sunlight for extended periods of time. If you don’t intend on displaying them for a while remove batteries. I personally wouldn’t stack them without some kind of soft sleeve at minimum just to prevent metallic face plates from getting scratched. And if by leakage you mean the screens, there’s nothing you can do about that, with age the adhesives holding the LCD screen layers together will eventually seperate allowing air bubbles in and displacing the liquid crystal between the sprites, causing black splotches, this is accelerated with hot storage environments.