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LaTeX provides commands \raggedright and \raggedleft, but none to cancel their effect. The \centering command is implemented in the same way as the \ragged* commands, and suffers in the same way.

The following code (to be inserted in a package of your own, or as internal LaTeX code) defines a command that restores flush justification at both margins:

\def\flushboth{%
  \let\\\@normalcr
  \@rightskip\z@skip \rightskip\@rightskip
  \leftskip\z@skip
  \parindent 1.5em\relax}

There’s a problem with the setting of \parindent in the code: it’s necessary because both the \ragged commands set \parindent to zero, but the setting isn’t a constant of nature: documents using a standard LaTeX class with twocolumn option will have 1.0em by default, and there’s no knowing what you (or some other class) will have done.

Any but a really old copy of Martin Schröder’s ragged2e package has a \justifying command to match its versions of the LaTeX ‘ragged’ commands. The package also provides a justify environment, which permits areas of justified text in a larger area which is ragged.