It doesn't matter what you use for the ID's as long as you are not repeating them in the same file.
I personally don't use anything that doesn't have a meaning. i.e.
I use things like:
Code:
<h2 id="ch01">Chapter One</h2>
For chapters.
Code:
<h2 id="pt01">Part One</h2>
For Parts.
Code:
<h2 id="bk01">Book One</h2>
For Books.
Code:
<h2 id="contents">Contents</h2>
For tables of contents.
etc.
This way they are all unique throughout the epub and I can instantly know what the ID will be for.