It depends on my mood. I never thoroughly learned how to type using the number pad on the right of the keyboard, so I'm always hunting the arabic numerals on top of the letter keys. If I deem it more difficult to hit the correct arabic digit key, then I will use roman numerals, because I actually know where those are.

It all comes down to how lazy I feel. Do I sacrifice a bit of speed and use romans, or do I sacrifice the comfort of turning my neck down and hunting out 1-0 on the keyboard?