The official rule I learned about writing number is that you have to write 1-20 in letters, then 30/40/50/60/70/80/90/100/200/etc in letters too. Same goes for round thousand numbers. Everything in between and higher can go in numbers.
At least, this is the rule I learned at Journalism School and which is a standard at all newspaper in the Netherlands. I can only assume that it also applies international, but I'm not sure about that. Anyway, it's the way I write numbers and I stick to that. And I'm known to drive people crazy by pointing that out while BRing.
Saskia
