I've sen my fair share of originals, and would guess that tapered rods were the norm.
But just why, I don't know.
I also doubt that there are many rifles today that have the original rods, but I've seen a few 'right out of the woodwork' guns that likely had original tapered rods. Certainly a lot of rifles have rods that were used during the working life of the gun, and would guess that rods were considered expendable due to breakage, and relitively easily replaced. I say considered expendable, because most rods don't look like they had a huge amount of effort put into their making. None seem to be perfectly straight, perfectly round, or perfectly tapered. Warpage could account for some of the lack of straightness, but a somewhat cock-eyed rod could also help hold it in place.
From lack of necessity, I've never made a rod by splitting it out of a piece of log, and wonder if the split would be such that a tapered rod would be the likely result? Also, if rods were considered expendable, it would be easier to make a tapered one, than one that is straight sided along the full length, espically if you're cutting the thing out in the woods with your pocket knife.
As for the ramrod channel and thimbles, as far as I see the channel are parallel and the thimbles all the same size, with no account given to the taper of the rod.
Tis a different time and place now, and maker/buyer opinions may vary.
John