Sorting round balls for competition/match shooting can make a difference if you follow Dutch's advice. Swaged, store-bought balls, I would sort into 0.1 grain groups and only use the top one-half for score. The others I would use for "barel-warmers" or very close-up shooting.
Sorting home-cast round balls assumes you have good casting technique, consistent melt temperature, and a quality mould. Smaller balls like .32. .36, & .40 are more sensitive to differences in weight, and larger; ,45, .50, & .54 less so. As with store bought, I sort them into groups of 0.1 grain. I usually end up with a few very light ones which I re-melt, a few very heavy ones (re-melt) and a bunch in the middle. For competitive shooting I would use the groups that weighed no less or more then 0.2 grain on either side of the center, average. When shooting for score, centering the sprue when loading home-cast balls helps to reduce the effect of minor differences in weight as any void that exists will usually be right under the cut-off mark.