No, not always. It should clear the cache on the server but the browser will usually need clearing as well. While one could clear their own browser cache, that doesn't mean that the cache for other users will be cleared. Renaming the skin will force all browsers to download the skin again because they will see it as a new set of files. That's why the Skin Version Guid is so nice, we can effectively change the url the css without needing to rename the actual skin directory.