Like Larry said, Atahualpa comes with *many* options that pretty much eliminate the need for child themes. ATA lets you make CSS modifications that will survive an upgrade. (and much more!)
That being said, I sometimes make Atahualpa child themes for the express purpose of having a functions.php file with some custom php in it that doesn't get overwritten during an Atahualpa theme upgrade.
It's also handy to put a editor-style.css file that lets you add some tweaks to the WordPress page/post editor appearance and keep them separate from the theme files.
Last edited by kdawes01; Nov 22, 2010 at 05:24 PM.