It doesn't matter, since the <p> tag only introduces a paragraph _break_.
Adding it where it really doesn't have to be just introduces some information
into your document that is redundant, but nontheless might clarify things for
the writer (or the reader). Browsers should be able to parse any use of the
<p> tag.
When describing this in a markup tag style guide, you might say that the
entire text is assumed one large paragraph of body text (brxdtekst), unless
marked otherwise or broken by <p> - paragraph delimiters.
Personally, I sometimes use <p> </p>, sometimes only <p>. Being inconsistent
in this way is probably the worst thing of all... :)
-- Torgeir @ http://www.ii.uib.no/~torgeir/- this summer @ http://www.nr.no/home/veimo/