Why do English writers avoid explicit numerals?
Based on my previous question regarding spelling out monetary amounts, is there a rule of thumb when a number should be spelled out?
These examples are easy:
I have two brothers.
It's exactly 823 miles to New York.
But do I have 15 minutes of fame, or fifteen minutes of fame?
Edit: This related question asks how the practice of spelling out numbers originated; I'm looking for specific guidelines.