\setuplayout[grid=yes]

\showgrid
\showframe

\definegridsnapping[whatever][strut,offset:-3pt]

\definehead[topsection][section]

\setuphead
  [section]
  [grid=strut]

\setuphead
  [topsection]
  [grid=whatever]

\starttext
    \topsection{test} test
    \section{test} test
    \section{test} test
    \section{test} test
\stoptext
