Always wear sunglasses. Always wear the sunglasses over the helmetstraps. I've heard that this has something to do with safety but I don't buy it. Just do it. See Spartacus for inspiration:
I'm not quite sure if this is universal or not, but clear yellow lenses = not cool in my book. Sure they have a purpose (riding at night), but riding at night isn't about looking cool, it's about getting home without getting run over, so ugly sunglasses at night are ok, but they're still ugly.
To those that think Team Performance brand (or other super cheap) sunglasses are "just as good" as the more expensive brands, you're wrong. They may protect your eyes but they won't protect your soul from me thinking you look like a goober. There's a reason designer jeans cost more than walmart ones. You can buy your jeans from anyplace you like, just don't get your sunglasses from Performance. Don't give me that "they're cheap and I'm broke" excuse either, I know your bike cost 4 grand. Get a decent pair of Oakley/Rudy/whatever looks decent sunglasses for another $60.
(Fabian pic from Saxo Bank Website)