Using your code snippet as an example, \centering only has an impact on the \caption. However,
\hspace*{\fill}
{ILLUSTRATION 1} \hfill {ILLUSTRATION 2}
\hspace*{\fill}
is meant to set ILLUSTRATION 1 and ILLUSTRATION 2 even distributed across \textwidth:
\documentclass{article}
\usepackage{showframe}% http://ctan.org/pkg/showframe
\begin{document}
\begin{figure}
\centering
\hspace*{\fill}
{ILLUSTRATION 1} \hfill {ILLUSTRATION 2}
\hspace*{\fill}
\caption{Example float with two illustrations}
\label{fig:mult1}
\end{figure}
\end{document}
However, replacing \hspace*{\fill} with \hfill, the space from the right margin is "gobbled":
\documentclass{article}
\usepackage{showframe}% http://ctan.org/pkg/showframe
\begin{document}
\begin{figure}
\centering
\hfill%\hspace*{\fill}
{ILLUSTRATION 1} \hfill {ILLUSTRATION 2}%
\hfill%\hspace*{\fill}
\caption{Example float with two illustrations}
\label{fig:mult1}
\end{figure}
\end{document}
giving value to the command definition for \hspace*:
LaTeX removes horizontal space that comes at the end of a line. If you don't want LaTeX to remove this space, include the optional * argument. Then the space is never removed.
An alternative to use if \hfill is more intuitive than \hspace*{\fill}, is to insert dummy text (like \null or \mbox{}) at the (start and) end of the line. That way the \hfill will always have "something to infinitely stretch against":
\null\hfill%
{ILLUSTRATION 1} \hfill {ILLUSTRATION 2}%
\hfill\null%
In the above examples, showframe merely highlighted the text block.
The code can be adapted very easily to not get the warning anymore:
\documentclass{minimal}
\usepackage{tikz}
\begin{document}
\noindent
\begin{tikzpicture}
\draw[line cap=rect] (0,0) -- (\linewidth-\pgflinewidth,0);
\end{tikzpicture}
\end{document}
EDIT: I think I also have an explanation: Each time tikz draws a line, it uses 2 bounding boxes (cf. PGF manual v 2.10 - page 579):
As a side-effect, the path construction commands keep track of two
bounding boxes. One is the bounding box for the current path, the
other is a bounding box for all paths in the current picture.
Section 7.13 details these parameters.
What I think is happening, is that regardless of the way how the line ends (line cap), its bounding box protrudes by \pgflinewidth beyond the edge of the text area by this length. This is probably necessary to accommodate the whole line in case of nicely rounded line caps. Since this requires in fact .5\pgflinewidth on each end and the bounding box of the line starts at exactly (0,0) the overhang will be \pgflinewidth at the end of the line.
Best Answer
\hsizeis the main parameter that TeX uses when typesetting: whenever it finishes a paragraph it looks at the current value of\hsizefor breaking it into horizontal boxes. Next, there are\leftskipand\rightskipand possibly other paragraph shape parameters (\hangindentand\hangafteror the more general\parshape).LaTeX uses an indirect approach and maintains many
\...widthparameters.\textwidthis generally the global width of the text area, while\columnwidthis the width of a column of text (it will be different from\textwidthwhen typesetting in two or more columns). However, inside aminipage,\textwidthwill be set to the given argument along with\hsize,\columnwidth, and\linewidth(they will revert to the previous values at the end of theminipagebecause it forms a group). Note that\parboxdoesn't reset\textwidth; the size is available as\linewidth.The parameter
\linewidthcontains the line length inside alist(or derived) environment and it may change in a nested list (while\hsize,\textwidthand\columnwidthdon't change).When we have to specify a length depending on current conditions, we have to use the correct parameter. For example, the width of a figure should be specified in terms of
\columnwidthin afigureenvironment and of\textwidthin afigure*environment; however this is done rarely when it's known that the document will be typeset in one column format. The same should be for atabular*ortabularxenvironment.Instead, when we need something centered with respect to a line in a
list, we should use\linewidth:In this case it would be wrong to use
\textwidthor\columnwidth, as the line length is "unknown" at typing time.Notice that LaTeX uses
\hangindentonly for typesetting sectional titles and\leftskipandrightskipfor\centering,\raggedrightand\raggedleft; the indentation of alistenvironment is obtained via\parshape.