I don't think it's safe to cut the shank on a bit. The difference between a loose and tight collet is a few thousandths of an inch, so it needs to tighten a full length shank to be securely held in. You don't want to be in the same space when a bit goes flying.
A shank can be cut without upsetting the balance of the cutter. As long as the cut is square. Because then there is no material off axis that will change the center of mass. think about it, a poor cut at an angle will have shifted the mass to the side of the shank that has more material relative to the axis, out of balance for sure, right?
I've cut shanks before, and as long as you cut & finish them square and NEVER cut them too short to not allow the collet to apply it's entire grip length to the shank, you'll be okay. I always leave one half diameter of length as a minimum length to be longer than the grip length of the chuck. I've never had a mishap. Always tighten your chuck to the manufacturer's specs. Beware of cheapo-depot cutters, because the shanks may not have a consistent diameter. And their balance is dubious anyways. If your cutter walks in the collet, something is not correct.
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