It makes sense to optimize the site’s cascading style sheets (CSS), as it pages load faster, and it reduce the amount of data transferred, and just combining css files could significantly reduce requests to server. So users benefit from faster page loads and webmaster of the sites benefits from the cheaper price of the transferred data.
I have used CSSTidy on many projects. And CSSTidy is very good tool for packing CSS and it can even fix CSS errors.
Browser cache is very useful when users download the same CSS and JS files multiple times. Some browsers, however, use the old CSS and JS files from the cache, even though they have been updated. This may lead to unpleasant situations, when the pages are displayed to the user with the wrong styles or pages will work incorrectly.
Fortunately, these unpleasant situations is easy to avoid and let the browsers cache files as long as they are changed.