One path or several paths for a tikzpicture?

Perhaps the title of my question is not fine (let me know).

I try to finish a document about the best practices when programming with TikZ. I remark in several answers that a lot of users like to use long path where nodes are created, edges are drawn (with edge- etc. The four next pictures shows the result that I want, then some mistakes with bad practices. If a rule could be given, what would it be? If you know of other ways to get the same kind of results, you can let me know.

Good results

If we look at the pgfmanual, specially the tutorial, a lot of codes look likes 1 but sometimes T. Tantau uses 3

Code for the first example :

\documentclass[11pt]{scrartcl}
\usepackage{tikz,tkzexample,fullpage} % tkzexample from TL2011
\usetikzlibrary{arrows}

\colorlet{graphicbackground}{blue!10}
\colorlet{codebackground}{red!10}

\begin{document}
\parindent=0pt
\thispagestyle{empty}

\begin{enumerate}
\item
\begin{tkzexample}[width=3cm]
\begin{tikzpicture} [every node/.style={blue,draw,circle}]
\node(a){a};
\node(b) at (2,1){b} ;
\draw[red,->] (a)  to[out=90,in=180] (b);
\end{tikzpicture}
\end{tkzexample}
\end{document}

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