Not wishing to dismiss the results of your experiments, but that assertion discounts the fact that conductors are cut to length after the sheath has been stripped.
I used to be sure that I once saw a video of one (one of those rolling promotional ones you sometimes see in hardware stores). It was fairly chunky, and basically you stuck the end of T/E into it, squeezed the handle, and it removed some sheath, stripped the ends of the L & N, and splayed the 3 cores out a bit.
But I've never seen anything like it for sale, and never known anybody else ever describe it, so either I imagined the whole thing or it was a failure.
Re stanley knives - one of mine with a retractable blade has a "setting" where on the first notch the very tip of the blade protrudes. You'd struggle to draw blood with it, but it is fantastic for drawing along the centre of T/E to score the sheath but not penetrate it, after which it can easily be torn. Or "cheesewired" without any risk of stretching.
They work fine for the sheath, watch the video in the Amazon link! I use the Irwin version. Works for flex, too. It would only take 1.25mm² flex initially, so I filed a bit off the jaw to make it do 1.5 also. Only downside is the slight bitemarks left behind...
...but that's a damn site neater than the mess left by the cheesewire and side cutters method IMHO...
Scoring round it like andy11 said solves that, but he must be one of very few who bother! And that just brings back up the "risk of slicing the insulation" debate, which is what I thought was the point of the cheese wire method for the sheath in the first place!!
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