In a perfect world the chamber throat diameter of a 357 revolver would be .358 and the barrel groove diameter would be .357. The critical point is that the groove diameter should be .001 smaller than the throat. Cast bullets should be .001 larger than the groove diameter or the same as throat diameter.
Of course, it's not a perfect world and the above relationships often get lost.
Over sized bullets will be less accurate because the rather abrupt sizing operation as they go through the throat and forcing cone is not as neat and tidy as doing it with sizing dies.
A few years ago Dan Wesson ran an estensive research project to determine best throat diameter, forcing cone angle, etc. An obvious thing to want to know, but nobody else bothered to run any experiments in the last 100 years.