I shoot a lot of cast bullets and I would not fire lap my 45/70's. They have always shot well without it. If you cast your bullets it is critical to pour them all at the same temp. or they won't weigh the same.
I try to get mine with-in .2 of a grain but if they are with-in 1 1/2 gr. I will still use them. Are you going to use gas checks?
I found that it works best to use them for any of my hot loads.