Respectfully, I gotta disagree on this one. I've been using that method for years on 12 and 14 gauge solid and I can count the nicked wires on one hand. The key is setting the depth right on your knife and using a light touch - you're not trying to hack through it. Plus with the price of decent strippers these days, I've had more issues with cheap ones damaging the insulation or leaving little burrs than I ever have with a quick score and snap. It's one of those things that sounds sketchy but works fine once you get the feel for it.