Seems to me the reason to trim them is to make them more uniform.  That said, trimming would be an extension of the manufacturing process.  I would trim first & weigh after.

Joe_S,

  Good advice so far, if you want to go that far. How far you go will depend on what type matches you plan on shooting. The longer the distance the more critical you want to be.  Personally, at the ranges I've shot the .45/70 at in match competition..... up to 500 yds...... I haven't seen that much need to be super critical on what's needed for accuracy. The .45/70 is just not capable of 1/2 MOA accuracy, or better, that would require going to extremes like the bench rest shooters do. What it really comes down to is what gives you confidence that you are making the best ammo possible.

  Personally I think load development is more critical in this caliber.

  What I do is first fire the cases and then size them enuf to hold the bullet properly and then trim them to the same length. But again, this is not super critical unless you plan on crimping your bullets, which you wouldn't want to do for best accuracy anyway.

  One of the more critical things to do would be to make sure you fire your brass in strict rotation. As the case is repeatedly fired and sized it will work harden and the neck tension will change which will have an effect on vertical accuracy. Some of the more finicky shooters will even go so far as to anneal their cases after every firing.

PETE