Closing them is mandatory but you are welcome to use the <P /> form. =P Since the DTDs are very clear that the P element only contains %Inline content, any other block type element automatically closes the P element, just like HTML. And doing so keeps you out of trouble trying to do things like:

<P>Blah blah
 <ul>
  <li />blarg
  <li />Blerg
 </ul>
</P> <-- bad bad bad! P can't contain other blocks.
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