I just did a Google search to find some of the rules for writing numbers and numerals and I was confused. Below are two links to a the ones that I felt most closely matched one another. I didn't know that so much thought could be put into spelling or not spelling numbers. Whoa!
After reading these articles, I still don't think that I know when to spell numbers out for people. Just reading the post by Richie Mae really got me thinking. There are some clear rules there, but there are more rules in the articles I read.
Also, thinking back on this same post, I went and looked at a math book that I still had and couldn't believe that the what Richie said was totally true. Math books are inconsistent in naming their naming of numbers.