Context is everything, IMO.

In engineering work, numbers should always be digits. In prose, numbers should be spelled out.

Breakfast at the Thompson's was a busy affair; 12 eggs and 6 rounds of toast for their 3 sets of boistrous twins.

Compared to

Breakfast at the Thompson's was a busy affair; twelve eggs and six rounds of toast for their three sets of boistrous twins.

To me it's pretty clear which of those reads better and more naturally as prose; digits really 'jump out' on the page, and while that is great for engineering texts, it is incongruent and distracting for prose.

There are exceptions to every rule. Sometimes it ends up being "between five and 15" which is psychotic.

Caveat, not including multipliers, like "273 billion"

You mean 273e9?

Any number that I write down is a number. I am not writing novels, the numbers I write down are supposed to be easy you find. You look through the document to find numbers, that is easy to do.

I spell out numbers when I want to emphasize them.

Take George Orwell for example:

"Nineteen Eighty-Four" has a lot more of a punch to it than just "1984."

00, 01, 10 there i did it

I somehow have "spell out if less than 13" burned in my mind from somewhere in middle school. No idea if it is right, but so far it has worked.

Nein

Spell out numbers under 10, but not when it's divisible by three or five.