<p>I'd say flag them for deletion on those threads that have a lot of comments &amp; answers&mdash;where clutter might be doing some harm&mdash;else leave them be.</p>

<p>So the policy would be this:</p>

<ul>
<li><p>Welcoming new users to the site is a good thing; so please feel free to do so in comments, even if there's nothing else to say.</p></li>
<li><p>There's no need for purely phatic comments to be preserved for posterity:</p>

<ul>
<li><p>Do delete your own once they've served their purpose, if &amp; when you happen to come across them again. Don't feel obliged to hunt them all down&mdash;you've got better things to be doing.</p></li>
<li><p>When a post is looking cluttered, flag them for moderators to delete. If it isn't really, don't do anything&mdash;moderators have also got better things to be doing.</p></li>
</ul></li>
</ul>

<p>I don't think precise criteria for when a thread is cluttered need to be established. When the flag queue's getting long, moderators can start to decline the "too chatty" flags they judge unnecessary.</p>
