Water ballast rookie question

I fly an ASW-28. The under-surface of the wing has a nice little piece
of plastic that covers the dump valve and which is hinged by a piece
of cloth tape. When flying with water ballast my assumption is that
correct procedure is to leave the piece of plastic in place, but open
and hanging down from the cloth tape hinge. I am thinking that airflow
will keep the plastic closed and be more aerodynamic over the dump
valve but when dumping ballast it will open sufficiently to let the
ballast dump efficiently. Or should I simply remove the piece of
plastic entirely when carrying water ballast? Thanks.
Tom