If I want skins to work on any base theme with minimal fussing, I believe I must get Tao's reset approach into every base theme I will eventually support (including core): declaring every core stylesheet file in the .info file — the Drupal CSS overwrite — but don't include any of the files. I might need to include Eric Meyer's reset as well, since it makes every browser behave the same.
No other Drupal 6 base theme I studied effectively resets core styles. Most of them re-set the styles with subtle changes in their 'base' or 'default' CSS files, not only screwing up any hope of a default base style but also making it very difficult to tame the specificity (although the cascade will always work). I have studied the latest Drupal 6 versions of Zen, Adaptive Theme, Omega and Fusion.
I have also studied the Drupal 7 versions of Zen, Tao and Omega (and found D7 allows for a far better core CSS removal approach). If there's any base theme or reset approach that I'd rather study before going Tao/Meyer, please put it in the comments.
BTW, I'm aware AJAX calls might insert core CSS back to the page (see bug at d.o). I don't know what to do about them, other than just removing the files from core (kitten blood all over my hands).
I'm diving deeper into Tao's stylesheets. It has a reset.css (based on Meyer's), a drupal.css that reimplements several core styles removing anything that gets in the way of good typography and a base.css that includes Tao's opinions on how things should be styled.
Nah, I'd rather mantain Tao's drupal.css than write my own.