What sort of comments do you add? Mine are usually of two varieties - either explaining a pixel value (width + border + margin), or adding a short note about why I did something unconventional (two pixels removed so FireFox doesn't flow to the next line). In either case, someone learning CSS wouldn't be hugely benefitted by them, since I don't make the comments all that verbose.
That said, I usually don't minify my CSS anyway. I was just wondering if there are any significant cases where comments would make a huge difference to someone who is learning CSS.
I think the latter is what could be useful to people learning.
E.g. "set X to Y so browser B doesn't do Z" is actually useful to someone who has never seen that behaviour and may encounter it, or be looking at you sources cause it seems you handled it somehow but it doesn't work on one's own page.
That said, I usually don't minify my CSS anyway. I was just wondering if there are any significant cases where comments would make a huge difference to someone who is learning CSS.