> given that optometrists tend to slightly overprescribe anyhow

I've been wearing glasses for myopia since I was ~10 years old and this is exactly opposite of my experience. Optometrists almost always prescribe the number slightly lower than they determine using tests.

When my number was constantly increasing, I remember ordering the next available increment just because the prescribed number was slightly annoying when looking at far away objects.

Now my number has been stable for a decade and I still sometimes get annoyed when using glasses at the prescribed power.