Ah, I just re-read the post and now I get it. But then that brings this:
You should be able to tell if there's no primer, either by checking them or when the powder flies out the flash hole and plugs up your press.
Each component has variances within themselves, similar to what Brian stated. My .308 cases have variances of over 5g between each one. Bullets are fairly consistent, but I have noticed .5-1gr difference between bullets in some of the bulk .223 and .40S&W I've bought. I think you'd have to figure out what's an acceptable variance to let slide, but at that point weighing finished ammo is probably a waste of time anyway.
Then again, maybe you have a system that works really well. I'd still love to see it.