fontstikz-styles
\bfseries is for bolding but what is for italics in tikz?
Example where italics needed for "Not SP graph".
\begin{tikzpicture}[scale=1]
\tikzstyle{every node}=[draw, circle, fill=white, inner sep=0pt,minimum size=0pt]
\draw (0,0) node(in){in}--(1,0) node{}--(2,0) node(b){b}--(5,0) node(e){e}--(6,0) node{}--(7,0) node(out){out};
\node[rectangle,font=\bfseries] at (3.5,-1.7) {Not SP graph};
\end{tikzpicture}
If you absolutely want to use something like this and you can use XeTeX or LuaTeX to compile your document, you can fake it for any font you like:
\documentclass{article}
\usepackage{fontspec}
\begin{document}
\fontspec[FakeSlant=-0.5]{Georgia Italic} hello?
\end{document}
I'm not sure if you'd call the output attractive, exactly:
The problem is caused by the empty line after you opened the tikzpicture environment. Note, that an empty line usually means a paragraph break, so don't use it in the document code just because of readability. If you would like to use (nearly) empty lines for separation, set at least a % into the line to comment it out.
It works this way:
\begin{tikzpicture}
[Karl's grid/.style ={help lines,color=#1!50},
Karl's grid/.default=blue]
Best Answer
The font styles in your document are defined by the class document or by a package or by you in the preamble or in your document. The font style used in the environment tikzpicture is the font style defined when you enter in the picture. You can change the style in the part of text in a node with
font= ...Like David wrote finally the font styles are defined in latex and Tikz do not change anything.