\documentclass{moderncv}
\moderncvtheme[blue]{classic}
\firstname{Some}
\familyname{Dude}
\title{CV}

\newif\ifdetails
\detailstrue
%\detailsfalse

\renewcommand*{\cventry}[7][.25em]{%
  \cvitem[#1]{#2}{%
    {\bfseries#3}%
    \ifthenelse{\equal{#4}{}}{}{, {\slshape#4}}%
    \ifthenelse{\equal{#5}{}}{}{, #5}%
    \ifthenelse{\equal{#6}{}}{}{, #6}%
    .\strut%
    \if\relax\detokenize\expandafter{\romannumeral-`\Q#7}\relax %\ifx&#7&%
      \else{\newline{}\begin{minipage}[t]{\linewidth}\small#7\end{minipage}}\fi}}
\begin{document}
\maketitle

\section{Using cventry:}
\cventry{year--year}{THIS IS JUSTIFIED, AS YOU CAN PLAINLY SEE}{THIS IS ALSO JUSTIFIED}{THIS IS JUSTIFIED, TOO}{AND THIS IS JUSTIFIED AS WELL}{%<- this one
\ifdetails
I would like all this stuff to be raggedright, however. The "year-year" is not important.
\fi
}
\cventry{year--year}{THIS IS JUSTIFIED, AS YOU CAN PLAINLY SEE}{THIS IS ALSO JUSTIFIED}{THIS IS JUSTIFIED, TOO}{AND THIS IS JUSTIFIED AS WELL}{%<- this one
\ifdetails
I would like all this stuff to be raggedright, however. The "year-year" is not important. %
\fi%
}

\end{document}