The only time I'll unload or shoot a muzzle loader (pull the plug for pellets or shoot for loose powder) is when I hunted a rainy day, wet snow or when the ML went from very cold to warm. I'd rather be safe than sorry and I sure was sorry one year when my ML went "frrrrrp" on a whitetail buck.
I tape the end of my ML and the projectile/sabot seals the forward end, my main concern comes from the breech which isn't sealed adding to the fact that propellants are hygroscopic (absorbs water from the air).
Edit: forgot to say that I leave my ML in outdoor temps all season when loaded. Longest I've left it loaded was well over a week with no issues but I care for it.