It could be argued that lines in a poem should not be coded as <p>, but maybe as <div> or <span> (with appropriate class and styling). Nowadays, I also prefer to have a separate <div> for each stanza:
Code:
<div class="poem">
<div class="stanza">
<div class="line"></div>
<div class="line"></div>
<div class="line"></div>
<div class="line"></div>
</div>
<div class="stanza">
<div class="line"></div>
<div class="line"></div>
<div class="line"></div>
<div class="line"></div>
</div>
</div>
and of course, titles and signatures will have their own <div class="title"> and <div class="signature">.