\setuphead[section][%
placehead=empty,
before=,
after=]
\setuphead[subsection][%
placehead=yes,
before={\blank[line]},
after={\blank[line]}]
\starttext
\showframe
\section{My section}
\subsection{My subsection (whitespace before: problem)}
\input knuth
\subsection{My subsection (whitespace before: fine)}
\input knuth
\page
\subsection{My subsection (NO whitespace before: fine)}
\input knuth
\subsection{My subsection (whitespace before: fine)}
\input knuth
\stoptext