Select a system and stick to it. One book in the For Dummies series has “10” and “ten” in the same paragraph!
Today's Wall Street Journal says: "And the bombs blew up within 3 miles of six level-one trauma units."
I don’t spell out numbers in addresses or prices, except for low numbers like “One Main Street” or “five bucks.”
When numbers are approximate and used to present a mood rather than data, I usually spell the number, as in: “The chairman was surprised when more than fifty people showed up for the meeting.”
"A million years ago" is assumed to be an approximation. If you type "1,000,000 years," people will slow down and notice the digits and may assume that the number is precise.
(from my upcoming No More Ugly Books!: design help for writers who don't hire artists)