I was told once, it mattered due to the side the tooling approached the work. I chose not to argue it, since the guy who said it was born the same year the pistol came into service?
Besides it was my Dad, and he'd of kicked my a..!
But,
It could very well have something to do with a taper on the sight dovetail as well, rather than the milling in the slide. The taper on the sight never being rammed all the way through the cut would have assured a secure interference fit, without the need for microscopic accuracy.