I have no doubt that weighing, sorting and throwing the bad-uns back in the pot will improve your groups. Eliminating or controlling the variables is exactly what muzzleloading accuracy is all about. Same goes for Archery, Golf, Ping Pong, Nascar, Knitting, Basket weaving and every other endeavor on God's earth. Historically speaking, the average 18th C shooter most certainly practiced good casting technique and threw the poorly cast ones back in the pot, and learned to keep the heat consistent etc.
If I was a competitive target shooter, you can bet I'd be weighing them, and doing whatever it took to wring the last bit of accuracy out of my riflegun. I tend to do much better at shotgun wing shooting than precision rifle shooting. Always been that way. Funny thing, my Dad was a good rifle shooter but could not hit a flying bird to save his life.
K