I leave the powder in the powder measure on my Dillon. I use the same powder for everything I load, so no danger of a mixup. The basement is cool, dry, and dark. No adverse effects so far. Been doing this for years, Just plain laziness, I guess. Actually though, the way I load I need to do this. I may get ten minutes here and there to load. Sometimes I may load as few as 25 rounds, go away, and later that day load another 50 or 100. May not get back for several days, may be back in an hour. Sure doesn't hurt anything to empty the measure, but depending on the circumstances it may not hurt anything to leave it.