How to neatly space the equals sign when using probabilities?

It's straightforward to define a macro such as \newcommand\myeq{\mkern1.5mu{=}\mkern1.5mu} -- choose the argument of \mkern to suit your personal preferences -- and thus to rewrite your equation as

 \Pr(A\myeq a)  &= \Pr(B\myeq b \mid C\myeq c)

In the TeXbook (p. 174, near bottom of page), though, DEK suggests not reducing the whitespace around the = symbols but, instead, adding more whitespace elsewhere in the full equation via judiciously-placed \, directives:

 \[ \Pr(\, A=a \,) = \Pr(\, B=b \mid C=c \,) \]

A full MWE:

\documentclass{article}
\newcommand\myeq{\mkern1.5mu{=}\mkern1.5mu}
\usepackage{amsmath}
\begin{document}
\begin{align*}
\Pr(A=a)       &= \Pr(B=b \mid C=c) \\           % original form
\Pr(A\myeq a)  &= \Pr(B\myeq b \mid C\myeq c) \\ % less whitespace around "="
\Pr(\, A=a \,) &= \Pr(\, B=b \mid C=c \,)        % *more* whitespace
\end{align*}
\end{document}

I turn my comment to an answer since I think it is an adequate solution, in particular in this situation, and it is simpler than the other approaches. I also find no flaw in its visual appearance. It also makes sense to me logically, since A=a can be viewed as a compound label for an event, not necessarily as an equation. Even if it's an equation, it's just too tiny to be granted the same rights as other equations.

\documentclass{article}
\begin{document}
\[ \Pr(A{=}a) = \Pr(B{=}b \mid C{=}c) \]
\end{document}

This reduces by half the space around the equals sign in the argument to \Pr; it also has the advantage of being able to set the size of the delimiters in an optional argument.

\documentclass{article}
\usepackage{mathtools}

\DeclarePairedDelimiterX\PrArg[1]{(}{)}{%
  \renewcommand\given{\Conditional{\delimsize}}%
  \begingroup\lccode`~=`= \lowercase{\endgroup\let~}\reducedequals
  \edef\equals{\mathchar\the\mathcode`= }%
  \mathcode`="8000
  #1%
}
\providecommand\given{}
\newcommand{\Conditional}[1]{%
  \nonscript\;#1\vert\nonscript\;\mathopen{}%
}
\renewcommand{\Pr}{\operatorname{Pr}\PrArg}
\newcommand{\reducedequals}{%
  \mskip-.5\thickmuskip
  \equals
  \mskip-.5\thickmuskip
  \nobreak
}

\begin{document}

$\Pr{A=a} = \Pr{B=b \given C=c}$

$\Pr[\big]{A=\frac{1}{2}} = \Pr[\Big]{B=b \given C=\sqrt{\frac{a^{2^n}}{b}}}$

\end{document}