The distance you sight your rifle should be determined by how far you intend to shoot, or what is the longest shot that you expect. The 45 70 has more of a rainbow trajectory than the faster bottleneck calibers. This means that if you wish to shoot more than 125 yards, you need to know the actual distance and also how much to hold over at that distance.
There is also the practical aspect of just how good your eyes are. Eyes need more and more help after age 50 or so. The aperture sights are more forgiving than the factory sights, and a bit more friendly to older eyes. Personally, I have increasing trouble with factory iron sights for distances beyond 50 yds. For me, the front bead covers nearly all the target at 100 yds. Because of this, I use a 1.75-5X scope for my older eyes.
If I were using irons, I would suggest you sight in for 2" high at 75 yards. This should give you a point blank aim point out to 125 yards. And it will be a bit easier to see the target, hold a bit finer bead, and assess group size at 75 yards than at 100. Try it yourself and see what you think. How much of the target you can see at 50, 75, 100, 125 yards? The aperature sight gives a larger peripheral sight picture, and is faster on target than the factory irons, and is more intuitive to use. Just put the front bead where you want it. No need to set it just right in the sight groove. Your eye will automatically center the front post in the rear aperture.
You should plan to shoot at 50, 75, 100, and 125 both to see where the bullets impact and determine the longest distance where you can reliably hit. If you can't consistantly hit a 10" paper plate at these distances, either hunt shorter, or mount a scope.