\documentclass[a4paper]{article}

\usepackage{pgf}
\usepackage[top=1cm,bottom=1cm,left=1cm,right=1cm]{geometry}

\begin{document}
\parskip=0pt
\parindent=0pt

\pgfkeys{/pgf/number format/every relative/.style={fixed}}

\def\TEST#1{\texttt{#1}$\to$\pgfmathprintnumber{#1}}

\long\def\TESTS[#1]{%
	\paragraph{#1=0:}
	\pgfkeys{/pgf/number format/#1={0}}
	\TEST{6.42e-16}\hspace{1em}
	\TEST{1.2}\hspace{1em}
	\TEST{6}\hspace{1em}
	\TEST{20.6}\hspace{1em}
	\TEST{12345678999.123}\hspace{1em}
	\TEST{87}\hspace{1em}
	\TEST{879}

	\paragraph{#1=1:}
	\pgfkeys{/pgf/number format/#1={1}}
	\TEST{6.42e-16}\hspace{1em}
	\TEST{1.2}\hspace{1em}
	\TEST{6}\hspace{1em}
	\TEST{20.6}\hspace{1em}
	\TEST{12345678999.123}\hspace{1em}
	\TEST{87}\hspace{1em}
	\TEST{879}

	\paragraph{#1=2:}
	\pgfkeys{/pgf/number format/#1={2}}
	\TEST{6.42e-16}\hspace{1em}
	\TEST{1.2}\hspace{1em}
	\TEST{6}\hspace{1em}
	\TEST{20.6}\hspace{1em}
	\TEST{12345678999.123}\hspace{1em}
	\TEST{87}\hspace{1em}
	\TEST{879}

	\paragraph{#1=3:}
	\pgfkeys{/pgf/number format/#1={3}}
	\TEST{6.42e-16}\hspace{1em}
	\TEST{1.2}\hspace{1em}
	\TEST{6}\hspace{1em}
	\TEST{20.6}\hspace{1em}
	\TEST{87}\hspace{1em}
	\TEST{879}

	\TEST{123456999}\hspace{1em}
	\TEST{123456000}\hspace{1em}
	\TEST{123456000.123}\hspace{1em}
	\TEST{12345678999.123}\hspace{1em}
}

\section*{Default settings:}

\TESTS[relative]


\section*{round mode=fixed:}
\pgfkeys{/pgf/number format/relative round mode=fixed}

\TESTS[relative]

\section*{fixed relative:}
\pgfkeys{/pgf/number format/fixed relative}

\TESTS[precision]

\end{document}

