Do full valence shells always result in a formal charge of zero?

A good place to start would be the ozone molecule, ${O}_{3}$. This is certainly a neutral molecule, but its common valence representation features a neutral $O$ atom bound to a formally positively charged $O$ atom, bound to a formal negatively charged $O$ atom, i.e. $O {=}^{+} O - {O}^{-}$. The central atom bears an undepicted lone pair. Why does the central $O$ atom bear a formal positive charge?