How to center column values in a table?

You have to load the package array, define a new column type with horizontal centering

\newcolumntype{P}[1]{>{\centering\arraybackslash}p{#1}}

and use this one (P) instead of p

\documentclass[conference]{IEEEtran}
\usepackage{array}
\newcolumntype{P}[1]{>{\centering\arraybackslash}p{#1}}

\begin{document}

\begin{table}
  \centering
  \begin{tabular}{|P{2.5cm}|P{2.5cm}|P{2.5cm}|}
    \hline
    Reconstruction strategy & aa          & bb( \%) \\ \hline
    Classic                 & 3342 voxels & 68 \%   \\ \hline
    VC                      & 4296 voxels & 87 \%   \\ \hline
    V m=7                   & 4745 voxels & 96 \%   \\ \hline
  \end{tabular}
  \newline\newline
  \caption{title}\label{tab1}
\end{table}
\end{document} 

Output:

If you also want vertical centering, use m instead of p:

\documentclass[conference]{IEEEtran}
\usepackage{array}
\newcolumntype{P}[1]{>{\centering\arraybackslash}p{#1}}
\newcolumntype{M}[1]{>{\centering\arraybackslash}m{#1}}

\begin{document}

\begin{table}
  \centering
  \begin{tabular}{|M{2.5cm}|M{2.5cm}|M{2.5cm}|}
    \hline
    Reconstruction strategy & aa          & bb( \%) \\ \hline
    Classic                 & 3342 voxels & 68 \%   \\ \hline
    VC                      & 4296 voxels & 87 \%   \\ \hline
    V m=7                   & 4745 voxels & 96 \%   \\ \hline
  \end{tabular}
  \newline\newline
  \caption{title}\label{tab1}
\end{table}
\end{document} 

Output:

If the width of the columns is not predefined, the most simple solution is to use c instead of p:

\documentclass[conference]{IEEEtran} 
\begin{document}
\begin{table}
 \centering
 \begin{tabular}{|c|c|c|}
 \hline
 Reconstruction strategy & aa          & bb( \%) \\ \hline
 Classic                 & 3342 voxels & 68 \%   \\ \hline
 VC                      & 4296 voxels & 87 \%   \\ \hline
 V m=7                   & 4745 voxels & 96 \%   \\ \hline
 \end{tabular}
\newline\newline 
\caption{title}\label{tab1}
\end{table}
\end{document}

Output:

A very simple solution seems to work!

use tabular and p{...} and add \hfil before cell text. it seems that the text become centered! as an example:

\begin{table}
    \begin{center}
        \begin{tabular}{c|p{2.5cm}|p{2.5cm}}
        \multirow{2}{*} {something in multirow} & \multicolumn{2}{c}{another in multicolumn} \\
        & \hfil$Q in kg/s$ & \hfil$Q in m^3/s$ \\
        \hline
        first & \hfil7.3 & \hfil inf \\
        second & \hfil3.2 & \hfil0.5 \\
        third & \hfil inf & \hfil0 \\
        \end{tabular}
    \end{center}
\end{table}