When to use serif and when to use sans serif font family

• presentations: sans-serif
• documents:
  • up to one/two pages: sans-serif or serif
  • more pages: always serif
  • header/titles maybe in sans-serif

I know the question is about typography (i.e., aesthetics), but maybe an overview over several scientific studies which compare the legibility of typefaces is also interesting:

Which Are More Legible: Serif or Sans Serif Typefaces?

Finally, we should accept that most reasonably designed typefaces in mainstream use will be equally legible, and that it makes much more sense to argue in favour of serif or sans serif typefaces on aesthetic grounds than on the question of legibility.

Since aesthetics cannot be determined scientifically, the only important reason to prefer one typeface over another is really your personal opinion—but often it helps to stick to traditions.

a good and detailed answer can be found also here - although it's talking about web design, I think these are good guide lines also for printed documents: http://webdesign.about.com/od/fonts/a/aa080204.htm