\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