It's proper to write out numbers from zero to ten, unless it's a person's age. Above that, use your discretion. If it doesn't seem weird, go for it.
I doubt anyone will pay attention, however. Remember that they read thousands of essays, and they won't be hunting down things like that, unless it jumps out at them.
I was always taught write out the number if it's less than ten and always write it out if it's the beginning of the sentence. May be more of a grammar style thing though.