Adding a taxonomy specific css file to certain pages
If you want to load certain CSS files based on taxonomy, maybe you could name your files according to taxonomy vocabularies. Just build the file name based on the vocabulary of a term.

$file = 'mymodule_taxonomy_' . $vocabulary . '.css';
drupal_add_css(drupal_get_path('module', 'mymodule') . '/' . $file);


If you have a node object you can use taxonomy_node_get_terms() to get the terms for a node and use this code to load a proper CSS file.
Then, with a vocabulary named cars you can name a CSS file mymodule_taxonomy_cars.css. That file can be loaded according to a vocabulary.