Numbers

I've read that the general accepted rule for writing numbers in prose is that if it's anything between one and nine, you write the word. If it's anything above that, you write it as a numeral, eg 56 instead of fifty-six. Some say it's because that's two words (or two words with a hyphen between them.) But even 10 has to be written with numerals. I don't know why, maybe because from 10 on it's a big number and people can't deal with spelling it?!

But in my head it always makes more sense to write all of one to twenty as words, because up to twenty, it's still a single word...