Hmm... I was checking out a FAQ of Chicago Manual of Style and they use the styles mixed up themselves. For example, they give one example in which they both use "twenty one" and "twenty-one". But, as it appears to me, it's most common to just put a space between every single unit (one hundred, six hundred, four hundred eighty four). So... until I find a resource which is more consistent, I'll do it this way.

Thank you for your hint, anyway.