Select a system and stick to it. One book in the For Dummies series has “10” and “ten” in the same paragraph!
One of my personal rules is to use the same style when numbers are nearby: “eight to twelve” or “8 to 12”—not “eight to 12.” However, to avoid confusion and misreading, I write “four 10-lb bags, not “4 10-lb bags.”
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.”
(from my upcoming No More Ugly Books!: design help for writers who don't hire artists)