On AKs in general, is there any benefit to drilling the holes vertically before installing the gas block?
Yes, it's easier. Especially if you first machine a little relief into the barrel, to settle the drill.
The drill won't "dance" around on the round barrel face, as it tends to if you use the gas blocks gas port as your guide, unless you use some kind of round stock between the drill and the gas port to minimize that "Dancing" of the drill.
That drill size if considerably smaller than the gas port in the gas block, so if your gonna' use the gas block as a guide, I'd go with some kind of round stock, temporarily installed in the gas blocks gas port, as I said.
Just my humble opinion, but, I think the angle hole would be better on the projectile passing by it, but the straight hole is just fine.