Dave from CH4D told me that the spec on a .357 swage die was that the min size is .357 and the max is .358. So, by that I am guessing they hold to .001 , most are probably better than that but that is the spec he gave me.
Now Corbin states in his literature that bullets of the exact diameter to slightly larger than normally shoot the best in most firearms. We are talking .0001 to around .0005 over sized. Most loads will not show increased pressure signs with the larger bullet either.