I'm going WAY out on a very trivial limb today, and spitting in the faces of the Cycling Fashion Police.
Sunglasses should be worn INSIDE your helmet straps. Yes, I said it. In boldface, no less. And yes, I know that pretty much every "here's how not to look like a newbie bike dork" article says otherwise. Even the otherwise brilliant BikeSnobNYC misses the boat on this one (though the rest of that post is pretty darn solid).
Here's the thing: When I'm wearing both of these accessories together, I want to be able to take each one of them on and off independently. Maybe some clouds rolled in and I need to tuck my sunglasses away. Or maybe I'm at a rest stop and want to pop off my lid to cool my bald spot. Straps in or out doesn't make one bit of difference in the first scenario -- you can get your sunglasses on and off with ease no matter where they are in relation to your helmet straps. But let's take that second scenario. You stop the bike, hop off, undo your helmet buckle, lift the helmet off your head, and -- SONOFAB_TCH! The straps knock your glasses off. Why? Because you listened to the Fashion Police instead of common sense. And you probably scratched your glasses to boot. Sure, you could have taken your glasses off first, but then you're taking the glasses off so you can get out of your helmet only to put your glasses back on. Why?
(Now that raises an interesting question: Do people who wear prescription glasses while riding go straps-in or straps-out? Because many of those folks need their glasses on all the time, and thus would probably want to be able to remove their helmet without taking their glasses off. I'll have to study that one.)
If someone can tell me a PRACTICAL, FUNCTIONAL reason for "straps in", I'm all ears. But until then, I'm letting my dork flag fly in the form of fluttering helmet straps, unfettered by sunglasses.
(Thank goodness someone's out there tackling the big issues, right?)