That is correct, you want to weigh them on the same plane, so to speak. So the rod beam is parallel in other words. Hopefully a machinist who does this for a living will chime in.
I made a fixture once to weigh rods, but I could not get repeatable results. Years later I found it was because the fixture was magnetized from me having used heavy magnets to hold parts of it together during the mock up phase!
OT- the fireworks thing got its start from me making black powder to shoot with. I tamped a trouser hangar tube (you know the cardboard tubes that are on the hangars that come from the dry cleaners to keep from creasing your pants) full of BP, taped the tube to a BBQ skewer and lit it off, and darned if it didn't fly! Was hooked ever since.
1914 Popular Mechanics illustration: