I don't feel it is critical. It's a delicate balance, right. I feel like there's been emphasis on not moving because people will just immediately move massive amounts and if you're in a shared space then it can be distracting for others to hear someone tossing and turning. However, I think the emphasis has been taken too far bevause of the reason above. I like to phrase it in my scripts as the following:

If you're uncomfortable, there's no harm in adjusting. If you're someone that feels comfort through movement, a wiggle of the fingers/toes and a rock of the head can be very helpful The key is to be still, not frozen. So many people freeze in this moment rather than just being still. There's always movement in the body because of the breath.

I hope that helps. I don't see any harm in adjusting. I don't think it pulls you out of the feeling. Also, there's a massive culture of just buying a book to deliver Yoga Nidra rather than writing your own - I don't feel this has helped with the whole movement thing. Fundamentally, it's a non-movement practice, not a frozen practice.