I consider it a good habit to always insert a \label right after you open a new section (and subsection and subsubsection), regardless of whether you think you will want to refer to it later. You won’t believe how often I see friends hardcoding section references because they are too “busy” to go back to add the \label. Of course, all is good as long as the section numbering doesn’t change… 
And just like using \cite, please be sure to always precede a \ref with a tilde to avoid having a section number in the beginning of a line.
Here is an example usage. To declare a label,
\subsection{Grid Minor and Treewidth}
\label{sec:grid-tw}
and to refer to that section,
As we have seen in Section~\ref{sec:grid-tw}, the existence of a $k-by-k$ grid minor in $G_A$ is inevitable.
I have inherited from the Emacs AUCTeX package the habit of prefixing all the (sub)sections by sec:. Similarly, all theorem-like labels are prefixed by thm:.
