All of the sight dovetails I've seen on factory made muzzleloaders made after the 1960's are of a constant width. They were milled with a dovetail milling cutter so they aren't tapered.
With this in mind, I say, it doesn't make any difference which side the sight is removed from.
edit:
With a Getz barrel, the dovetail was made by a craftsman. He may have used a mill cutter or cut it by hand.
When cutting a dovetail by hand, I try to keep the sides parallel, like a milling machine cutter would make. In other words, I don't intentionally make a tapered dovetail.