Ack! Personally I'd save my money and just buy the set. It's much cheaper that way really. Between shipping costs and increased needle costs for singles you may be paying double or more for the equivalent set of needles.
That said- Some people use primarily tiny, some use primarily big. It really depends on what yarn weight is your preference and what projects you like to make. I prefer worsted and DK so my most often used needles are US 5,6,7,8. I do use other yarns and needle sizes though rarely anything over 9.