gunpowder can degrade with moisture. before the advent of methyl diphenyl urea and diphenlamine (ya i had to get the old text books out for this one) in gunpowder, nitroglycerine could be easily blended in with the nitrocellulose base rather then acting as a stabilizer if the gunpowder was kept in a damp enviroment. in a well sealed area like a casing you wont need to worry about this.
the primer is what you really need to worry about in old ammo. primer compounds of yester year degraded in any oxygen rich enviroment, this included loaded casings. to counter this various preservatives were used like salt and mercury. boxer primers commonly used mercury where as berdan commonly used salt. these primers are known as corrosive. a later developement seen the use of abrasives like glass and emery sand. these later primer compounds did not degrade but they required more internal friction to ignite hence the use of glass and sand. these primers were commonly called corrosive by american ammo makers but the primers were actually erosive. these primers were also the shortest lived. then came the awsome primers we know today using no preservatives, no abrasives and no mercury fulminate. they use a hardened yet highly ductile anvil to create a "blast plate" in the center of the primer to yield high ingition tempature, fast burn rates and a minimized deflagration effect.