A family of Microsoft word processing software products for creating web, email, and print documents.
For roman numerals, it's preferable to have the number aligned right. When you change from Left to Right in the Define New Multilevel List dialog, Word will adjust the indents and tabs automatically, but you can fine-tune them as needed.
You don't have to use heading styles to create multilevel lists, though if you are numbering headings, there are advantages to using the built-in styles (see http://shaunakelly.com/word/numbering/UseBuiltInHeadingStyles.html). If you do use the heading styles, you don't have to settle for Microsoft's formatting: you can modify the font and paragraph formatting to your own requirements; see http://shaunakelly.com/word/styles/ModifyAStyle.html.