You're going to get a variation in grain weight in any cast projectile - doesn't matter if it's round or conical. There can be a lot of variables involved as well - what you are casting, temperature of molten metal, temperature of mold, the purity (how well fluxed) the metal is, etc.
I've know fellows who go to the point of weighing every ball / conical and powder charge as well (I'm talking BP) - in the end, they usually weren't any better shooters than those next to them who shot their balls / conicals "as cast" and measured their powder by volume in a simple measure.
I have also seen informal rifle matches between fellows as I describe above who were very anal about the wts., etc and fellows who loaded out of the bag with a volume measure and round balls that were cast in a "bag mold" - with the sprue cut off with the mold handle - much more "primitive" than the nice sprue that a modern style mold makes. The guy loading out of the bag usually came out on top.
Given all the "careful" preparation of weighing ball weight and exact powder measurement - that is still not going to guarantee that you won't have a "flyer" once in a while - even with all of that, fouling and climate changes can affect things as well. As can a person's state of mind or physical being - especially if he partied the night before.
I cast .375 balls for my Navies - pure lead or as pure as possible. I cast 5 different conical type lead bullets for my 38 specials that I shoot a lot. Grain weight varies on all of them - but they all seem to shoot as accurately as the next one. While you can try to get everything "perfect" as far as ball weight and charge weight/volume - you have to also remember that while all chambers in a revolver are supposed to be "created equal" - they often are not. Throat size can vary even though they are within "specs." and some chambers will shoot better than others.
On cartridge revolvers (and since this is a BP forum, I'll consider we are talking about BP cartridge revolvers) - chamber throats can vary from one to another. In fact, shooting lead conical shaped bullets - a throat size may possibly be say .356 that the bullet has to go through and into a forcing cone that ends up in a barrel that's .357 + or -. This happens not only on revolvers but on long guns as well (cartridge). It doesn't matter what the grain weight difference is from one conical lead bullet to the next if the bullet is smaller in diameter than the bore is - you're going to end up "key-holing" or leading the bore usually (if using cast).
If a person wants to be picky about the grain weight of their balls or powder grain wt / volume of their BP charges - that's entirely fine. It's a hobby and some folks focus on such things - more power to them especially if they can "measure" the amount of accuracy that is improved by doing so. I am not against anyone wanting to be consistent or their focus on certain aspects of shooting. I just don't feel that unless a firearm is clamped in a vise so that it remains in the exact same position and you can insure that every ball / bullet is the same weight and the powder charge is exactly the same - that you are going to be able to really measure an improvement in accuracy. All things being equal - if a handgun is clamped in a vise and everything is the "same" - in theory, after six shots there should only be one hole in the target.
While it's fine to always want to improve you shooting accuracy and enjoy the various aspects of it - whether it be loads, bullet styles etc. - just remember that for any pistol shooter, things can vary your performance from day to day. The way you feel, a slight change in your hand grip, the ground you are standing on or a slight change in your shooting position. So . . . is the difference in accuracy from say two balls with a 1 grain variation or is it from you, the shooter?
I guess what I'm trying to say is - and this goes for new as well as old shooters - it's fine to follow what you want to follow and what your particular interests are - whether it be loads, casting, ball weights, etc. Just don't get too hung up on the "small things" so that you can't enjoy the shooting end of it and making smoke. I can assure you that our ancestors didn't get hung up on ball weight and such things in their everyday shooting - their concerns were that the gun went off if needed for protection or it put meat on the table.