I've frequently had this problem. There seems to be something in the way that key PointName is implemented in pst-eucl that's getting in the way.
The way \pstGeonode is setting the node names is through an artifact of how \psnode works---which is expecting something expandable in \edef to create string for the internal name for the node. When you're formatting the text with something like \textbf, you're creating something that won't work as a node name.
But the good news is that there is a reasonable work-around:
\pstGeonode[PointName=none](1,-2){C}
\rput([nodesep=1em,angle=-90]C){\textbf{C}}
So, the MWE is now:
\documentclass{article}
\usepackage{pst-eucl}
\begin{document}
\begin{pspicture}[showgrid=false](-4,-4)(4,4)
\pstGeonode[PointName=none](1,1){A}
\uput[135](A){\texttt{A}}
\pstGeonode[PointName=none](1,-1){B}
\uput[-45](B){{\scriptsize B}}
\pstGeonode[PointName=none](1,-2){C}
\uput[-90](C){\textbf{C}}
\end{pspicture}
\end{document}
Output:

There are actually two different syntaxes which work here:
You can use \rput and pass key-values:
\rput([nodesep=1em,angle=135]A){\textbf{A})
But pstricks provides a better vehicle for applying labels to nodes:
\uput*{labelsep}[refangle]{rotation}(x,y){stuff}
where the labelsep and rotation arguments may be omitted. In place of the coordinates (x,y) you may substitute the node name (A).