should xml:id be inside TITLE or SECT1?

I am creating a book mainly for PDF rendering but also for
ebook/chunked html output.

I'll be including cross-references throughout; I don't expect to do a
lot of fancy customizations.

Is there a preferred method for when to use xml:id?

<sect1 xml:id="wifi">
<title> </title>
<para>......</sect1>

OR

<sect1>
<title xml:id="wifi">Wi Fi Networking</title>
<para> </para> </sect1>

Does it make a difference? Are there advantages to putting it in the
title rather rather than the sect1 element? Opinions?  Thanks...

rj
-- 
Robert Nagle
htpt://www.robertnagle.info