OK, this isn’t a make-or-break item, but I find myself explaining it often enough that it’s worth putting my thoughts down for students once and for all.
First, I find that most of the so-called “rules” most of us were taught in high school (myself included) are nonsense. Each academic discipline–not to mention the publishing world, journalism, and various sectors of the Web, just to name a few–have their own conventions. The most common convention in my own discipline, MLA, tells us to spell out numbers that can be written as one or two words. If it takes three words or more, you can write it using numerals. So ten thousand is written in words, while 348 is written using numerals because it would take more than two words to write it out.
Naturally, there are exceptions. It’s usually a bad idea to begin a sentence with a numeral. Instead, try re-wording the sentence so it doesn’t begin with the number. Certain numbers that always go with a label are also OK to write as numerals.