This is one of those questions that the answer always starts of, "That depends,".
For very serious target shooting, I would hold it to +/- 1/2 grain. For more normal shooting and hunting open the limit up to +/- 1 grain. Much lower than -1 grain and you have to consider that you have a void. Larger bullets of say 54 caliber the limits can open up. The way I shoot, if the ball is smooth and I see no dimple at the sprue its good enough.