Yes. While combining minification and gzip doesn't produce as dramatic an improvement as either does above plain text, combining them does usually yield some additional improvement (especially if minification handles things like variable renaming in JS or property folding in CSS).

From experimentation using various "typical size" CSS, you can get a few more kb, which works out to about 3-7% extra savings for these file sizes.

Definitely worth it, particularly for high traffic sites.