Cutter angle is important when deciding to go past the neck-shoulder (N-S) junction into the shoulder itself. If we're turning into a 30° shoulder for example, with a 30° cutter, it's feasible to clean up the entire shoulder without weakening the N-S junction, which isn't to say it's good, or even OK to do so. On the other hand, turning into that same shoulder with a 90° cutter is high risk; even a tiny amount of overshoot and the neck may fall off the shoulder
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I've found the 21st Century cutter does an excellent job; their 35° cutter for use with 30° shoulders provides a good balance, it allows a visual inspection to serve as a sort-of GO, NO-GO gage with respect to shoulder turning.
One way to learn if a shoulder cut is in-spec, whatever that may be, is to long-section a turned piece and inspect the remaining junction, this may be compared to a long-sectioned, unturned piece as a zero reference.
Shoulder cutting begs the question, why? For myself, it gives the inevitable donuts a nice place to call home
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Anyway, here's a picture showing a couple long-sectioned pieces and my 21st Century cutter in it's lathe turning holder, I felt the top piece went a bit too far, the piece below was just about right. I've since adjusted my angle to around 32° so I can cut a bit more into the shoulder without weakening the N-S junction.
Outside view:
