What a lot of folk lore !!
The chances of a Mag producing a spark when the prop is turned backwards are nill if the prop is turned by hand at slow speed. The reason for this is that the impulse unit wont work, in fact to check the timing you HAVE to turn the prop backwards to dis-engage the impulse otherwise the timing will be set incorrectly.
So the bottom line is that each time the igniton timing is set the engine HAS to be turned backwards, I don't see vac pump failures after each 150 or annual check so it would seem that the thing about turning the vac pump backwards slowly is just so much hogwash.
If the vac pump was turned backwards at high speed it would be another matter.
But please remember Props Bite