After reading a number of threads on various forums, I've concluded that it is far easier to cut roughly with the 45 degree tool, leaving shavings which can damage the projectile when seating it. In an extreme case, the sloppy chamfering was so rough in a .223 that the user tore lumps off the projectile & severely marked the ogive with the seating plunger, such was then necessary seating force.
I guess you could remedy that shortcoming by polishing the neck inside after chamfering, but that would seem to be a tad tedious & imprecise to me. I'm happy to take my chances with my K & M & a minimum cut enough to just break the 90 degree shoulder. After all, the boattails I use don't take much encouragement to get past the case mouth.