Euro sign in table with siunitx

I would like to suggest an alternative solution: put the euro sign in the column headers.

And egreg suggests to use eurosym without the gen option, this way the euro symbol is more beautiful.

\documentclass{article} 
\usepackage[ngerman]{babel}
\usepackage[locale = DE]{siunitx}
\usepackage{eurosym}
\usepackage{booktabs}
\begin{document}    
\begin{table}[h]
    \centering
    \begin{tabular}{SSSS}  
    \toprule    
    {A (\euro)} & {B (\euro)} & {C (\euro)} & {D (\euro)} \\ 
    \midrule
    -199,37  & 0,00  & 33,27  & 166,10  \\
    252,56  & 0,00  & -409,58  & 157,02  \\
    104,67  & 0,00  & -40,18  & -64,48  \\
    0,17  & 0,00  & -2,52  & 2,35  \\
    0,00  & 0,00  & 0,00  & 0,00  \\
    \bottomrule    
    \end{tabular}
\end{table}
Or:
\begin{table}[h]
    \centering
    \begin{tabular}{SSSS}  
    \toprule    
    {A} & {B} & {C} & {D} \\ 
    {(\euro)} & {(\euro)} & {(\euro)} & {(\euro)} \\ 
    \midrule
    -199,37  & 0,00  & 33,27  & 166,10  \\
    252,56  & 0,00  & -409,58  & 157,02  \\
    104,67  & 0,00  & -40,18  & -64,48  \\
    0,17  & 0,00  & -2,52  & 2,35  \\
    0,00  & 0,00  & 0,00  & 0,00  \\
    \bottomrule    
    \end{tabular}
\end{table}
\end{document}

What about this?

\documentclass{article}
\usepackage[ngerman]{babel}
\usepackage[T1]{fontenc}
\usepackage[locale = DE]{siunitx}
\usepackage{textcomp}
\usepackage{booktabs, makecell}
\newcommand\mc[1]{\multicolumn{1}{c}{#1}}

\begin{document}

\begin{table}[h]
    \centering
\setcellgapes{3pt}
\makegapedcells
\sisetup{table-space-text-post={\,€}, 
         table-align-text-post=false}
    \begin{tabular}{S[table-format=-3.2]<{\,€}
                    S[table-format= 1.2]<{\,€} 
               *{2}{S[table-format=-3.2]<{\,€}}
                    }
    \toprule
\mc{A}  & \mc{B} & \mc{C} & \mc{D}  \cr
    \midrule
-199,37 & 0,00  &   33,27  & 166,10 \cr
 252,56 & 0,00  & -409,58  & 157,02 \cr
 104,67 & 0,00  &  -40,18  & -64,48 \cr
   0,17 & 0,00  &   -2,52  &   2,35 \cr
   0,00 & 0,00  &    0,00  &   0,00 \cr
    \bottomrule
    \end{tabular}
\end{table}

\end{document} 

With the appropriate table-format option for the individual cells as well as with table-space-text-post=€ and table-align-text-post=true you can get the following output:

\documentclass{article} 
\usepackage[ngerman]{babel}
\usepackage[locale = DE]{siunitx}
\usepackage[gen]{eurosym}

\begin{document}    

\begin{table}[h]
\sisetup{table-space-text-post=€, table-align-text-post=true}
    \centering
    \begin{tabular}{|S[table-format=-3.2]|
                    S[table-format=1.2]|
                    S[table-format=-3.2]
                    |S[table-format=-3.2]|}  

    \hline
    {A} & {B} & {C} & {D} \\ 
    \hline
    -199,37 € & 0,00 € & 33,27 € & 166,10 € \\
    252,56 € & 0,00 € & -409,58 € & 157,02 € \\
    104,67 € & 0,00 € & -40,18 € & -64,48 € \\
    0,17 € & 0,00 € & -2,52 € & 2,35 € \\
    0,00 € & 0,00 € & 0,00 € & 0,00 € \\
    \hline

    \end{tabular}
\end{table}
\end{document}

To avoid repeating the € symbol over and over again, I'd either put it into the column header of the corresponding columns or even into the caption of the table.