Prevent titles at bottom of page—sometimes

Question

I have a book that contains both prose and poetry. I don't want a poem to start too close to the bottom of the page.

That's what the nobottomtitles option of titlesec is for. I don't want this to apply to the prose sections, however.

A solution would be one of the following:

• Let nobottomtitles apply only to \raggedbottom environments.
• Allow nobottomtitles to be turned off or on. (Can the package option be changed midway through the document?)
• Let nobottomtitles apply only to a particular section level—say, subsubsection.

Example to work with:

\documentclass[12pt]{book}

\usepackage{verse}
\usepackage[nobottomtitles]{titlesec}
\usepackage{lipsum}

\setlength{\parskip}{0pt}

\newcommand{\poem}{%
\begin{verse}
First line of poem\\
Second line of poem\\
Third line of poem\\
Fourth line of poem
\end{verse}
}

\begin{document}

% Flush bottom for prose
\flushbottom

\section{Prose section}
\lipsum

\subsection{Prose subsection}
Can be at bottom of page

\subsection{Prose subsection}
Can be at bottom of page \lipsum

% Ragged bottom for poetry
\raggedbottom

\subsection{Poetry subsection}
\begin{verse}
Make\\
this\\
one\\
six\\
lines\\
long
\end{verse}

\subsection{Poetry subsection}
\poem
\poem
\poem

\subsection{Poetry subsection}
\poem
\poem
\poem

\end{document}