During Christmas, I took a percussion cap muzzle loader out to let my friends shoot it. I got a minie ball stuck halfway down the bore, and got it firmly seated at home once broke out a hammer. Since my local range prohibited use of black powder, I didn't shoot it for six months. It took two caps to fire when I finally discharged it, but it also took two caps to fire subsequent rounds until I cleaned the nipple. Keep in mind my state is constantly at 70%+ humidity.
I'd say leaving it loaded is fine.