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.”
Don't start a sentence with a figure, like "9,423 people registered to vote in August." To avoid spelling out "Nine-thousand, four-hundred, twenty-three" you can restructure the sentence as "More than 9,000 . . . ." or "The number of August registrants was 9,423."
(from my upcoming No More Ugly Books!: design help for writers who don't hire artists)